If ksmserver couldn't start, there is something low-level fundamentally wrong going on.

Easy things to check:
* did you install anything new recently (since things were working normally)?
* does your disk have ample free space (ie, not full)?
* anything noteworthy logged in ~/.xsession-errors ?

Here are my answers to your questions:

* No I didn't install anything new, only the usual updates.
* My hard drive has 283GB of free space in  /home  and 184GB in  /  I guess that's enough free space. 
* Troubles started when Kmail got stuck retrieving emails.  I stopped it but Akonadi kept working even when I logged in Cinnamon.  I deleted the folders  ~.kde, ~local/share/akonadi  and a hidden folder with the half downloaded mails in ~local/share 


Xsession errors shows this:
cinnamon-session[1171]: WARNING: t+0.32897s: Could not parse desktop file /etc/xdg/autostart/korgac.desktop: Invalid key name: X-KDE-autostart-condition[$e]
cinnamon-session[1171]: GLib-GObject-CRITICAL: t+0.32917s: object CsmAutostartApp 0x5608a9f42f70 finalized while still in-construction
cinnamon-session[1171]: GLib-GObject-CRITICAL: t+0.32921s: Custom constructor for class CsmAutostartApp returned NULL (which is invalid). Please use GInitable instead.
cinnamon-session[1171]: WARNING: t+0.32923s: could not read /etc/xdg/autostart/korgac.desktop
cinnamon-session[1171]: WARNING: t+0.45504s: Could not parse desktop file /etc/xdg/autostart/kgpg.desktop: Invalid key name: X-KDE-autostart-condition[$e]
cinnamon-session[1171]: GLib-GObject-CRITICAL: t+0.45561s: object CsmAutostartApp 0x5608a9f62ae0 finalized while still in-construction
cinnamon-session[1171]: GLib-GObject-CRITICAL: t+0.45608s: Custom constructor for class CsmAutostartApp returned NULL (which is invalid). Please use GInitable instead.
cinnamon-session[1171]: WARNING: t+0.45651s: could not read /etc/xdg/autostart/kgpg.desktop
** Message: couldn't access control socket: /run/user/1000/keyring/control: No such file or directory
** Message: couldn't access control socket: /run/user/1000/keyring/control: No such file or directory
** Message: couldn't access control socket: /run/user/1000/keyring/control: No such file or directory
SSH_AUTH_SOCK=/run/user/1000/keyring/ssh
=== xinerama setup Configuration ===
  Clone: false
  Output: Laptop attached to LVDS1
     status: on
     width: 1366
     height: 768
     rate: 60
     primary: true
     position: 0 0
  Output: (null) attached to DP1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
  Output: (null) attached to TV1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
  Output: (null) attached to VGA1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
  Output: (null) attached to VIRTUAL1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
=== Applying Configuration Configuration ===
  Clone: false
  Output: Laptop attached to LVDS1
     status: on
     width: 1366
     height: 768
     rate: 60
     primary: true
     position: 0 0
  Output: (null) attached to DP1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
  Output: (null) attached to TV1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
  Output: (null) attached to VGA1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
  Output: (null) attached to VIRTUAL1
     status: off
     width: -1
     height: -1
     rate: -1
     primary: false
     position: -1 -1
cinnamon-session[1171]: Gtk-WARNING: t+5.01652s: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found
cinnamon-session[1171]: Gtk-WARNING: t+5.12090s: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(cinnamon-settings-daemon:1374): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(cinnamon-launcher:1476): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(cinnamon:1505): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(canberra-gtk-play:1513): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found
Failed to play sound: File or data not found

(nm-applet:1524): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(abrt:1583): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(polkit-gnome-authentication-agent-1:1635): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found
Window manager warning: Log level 16: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(cinnamon-killer-daemon:1634): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(nemo:1620): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found
Initializing nemo-image-converter extension

(evolution-alarm-notify:1514): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

** (nemo:1620): WARNING **: Can not determine workarea, guessing at layout
Cjs-Message: JS LOG: About to start Cinnamon
(uint32 1,)
St-Message: cogl npot texture sizes SUPPORTED
Cjs-Message: JS LOG: Cinnamon started at Sun Jan 24 2016 08:32:19 GMT-0300 (UYT)

(cinnamon:1505): St-WARNING **: percentage lengths not currently supported
Cjs-Message: JS WARNING: [/home/albatros/.local/share/cinnamon/applets/timer-notifications/applet.js 379]: AppOptions is read-only
Cjs-Message: JS WARNING: [/home/albatros/.local/share/cinnamon/applets/timer-notifications/applet.js 380]: AppOptions is read-only
Cjs-Message: JS WARNING: [/home/albatros/.local/share/cinnamon/applets/timer-notifications/applet.js 380]: useless expression
Cjs-Message: JS WARNING: [/home/albatros/.local/share/cinnamon/applets/timer-notifications/applet.js 381]: AppOptions is read-only
failed to create drawable
Cjs-Message: JS LOG: Invalid network device type, is 14

(parcellite:1526): Gdk-CRITICAL **: IA__gdk_window_thaw_toplevel_updates_libgtk_only: assertion 'private->update_and_descendants_freeze_count > 0' failed

(cinnamon:1505): Cjs-WARNING **: JS ERROR: Error: Argument 'data' (type utf8) may not be null
soupQueue@/home/albatros/.local/share/cinnamon/applets/weather@mockturtl/applet.js:366


(cinnamon-screensaver:1833): Gtk-WARNING **: Theme parsing error: gtk-widgets.css:64:17: Theming engine 'unico' not found

(tracker-extract:1577): Tracker-WARNING **: Task 0, error: Unable to insert multiple values for subject `urn:uuid:b7aeafaa-9330-c2c7-56f3-7b200e6c127a' and single valued property `dc:description' (old_value: 'Would Iron Maiden ever play with an orchestra? "Just doing orchestral backing music to a rock track is utterly pointless. It's just kind of musical narcissism."', new value: 'Would Iron Maiden ever play with an orchestra? "Just doing orchestral backing music to a rock track is utterly pointless. It's just kind of musical narcissism." Boston's Classic Rock')
Sparql was:
INSERT {
GRAPH <urn:uuid:472ed0cc-40ff-4e37-9c0c-062d78656540> {
<urn:uuid:b7aeafaa-9330-c2c7-56f3-7b200e6c127a> nie:dataSource <http://www.tracker-project.org/ontologies/tracker#extractor-data-source> .
<urn:uuid:b7aeafaa-9330-c2c7-56f3-7b200e6c127a> a nfo:HtmlDocument ;
	 nie:keyword "vibNews" ;
	 nie:description "Would Iron Maiden ever play with an orchestra? \"Just doing orchestral backing music to a rock track is utterly pointless. It's just kind of musical narcissism.\"" ;
	 nie:keyword "music" ;
	 nie:keyword "iron maiden" ;
	 nie:description "Would Iron Maiden ever play with an orchestra? \"Just doing orchestral backing music to a rock track is utterly pointless. It's just kind of musical narcissism.\" Boston's Classic Rock" ;
	 nie:title "Bruce Dickinson Talks Iron Maiden’s 18 Minute Epic ‘Empire of the Clouds’ « 100.7 WZLX" ;
	 


(tracker-extract:1577): Tracker-WARNING **: Task 1, error: Subject `urn:uuid:86bd3546-59cd-4c66-f8dc-1229e5dbff65' is not in domain `nco:Contact' of property `nco:representative'
Sparql was:
INSERT {
GRAPH <urn:uuid:472ed0cc-40ff-4e37-9c0c-062d78656540> {
<urn:contact:%3Cpicasastamp%2F%3E%0A> a nco:Contact ;
	 nco:fullname "<picasastamp/>\n" .
}
}
INSERT {
GRAPH <urn:uuid:472ed0cc-40ff-4e37-9c0c-062d78656540> {
<urn:contact:Picasa%202.0> a nco:Contact ;
	 nco:fullname "Picasa 2.0" .
}
}

INSERT {
GRAPH <urn:uuid:472ed0cc-40ff-4e37-9c0c-062d78656540> {
<urn:uuid:86bd3546-59cd-4c66-f8dc-1229e5dbff65> nie:dataSource <http://www.tracker-project.org/ontologies/tracker#extractor-data-source> .
<urn:uuid:86bd3546-59cd-4c66-f8dc-1229e5dbff65> a nfo:Image ;
	 a nmm:Photo ;
	 nfo:width 350 ;
	 nfo:height 346 ;
	 nmm:dlnaProfile "JPEG_SM" ;
	 nmm:dlnaMime "image/jpeg" ;
	 nco:representative <urn:contact:%3Cpicasastamp%2F%3E%0A> ;
	 nco:creator <urn:contact:Picasa%202.0> .
}
}



(tracker-extract:1577): Tracker-WARNING **: Task 2, error: Unable to insert multiple values for subject `urn:uuid:8efcd2f2-f154-0f78-5d4e-0fcfbd304ad7' and single valued property `dc:description' (old_value: '
ESA’s XMM-Newton has found a wind of high-speed gas streaming from the centre of a bright spiral galaxy like our own that may be reducing its ability to produce new stars.
', new value: 'space')
Sparql was:
INSERT {
GRAPH <urn:uuid:472ed0cc-40ff-4e37-9c0c-062d78656540> {
<urn:uuid:8efcd2f2-f154-0f78-5d4e-0fcfbd304ad7> nie:dataSource <http://www.tracker-project.org/ontologies/tracker#extractor-data-source> .
<urn:uuid:8efcd2f2-f154-0f78-5d4e-0fcfbd304ad7> a nfo:HtmlDocument ;
	 nie:description "\nESA’s XMM-Newton has found a wind of high-speed gas streaming from the centre of a bright spiral galaxy like our own that may be reducing its ability to produce new stars.\n" ;
	 nco:creator [ a nco:Contact ;
	 nco:fullname "esa"] ;
	 nie:description "space" ;
	 nie:keyword "Space science" ;
	 nie:keyword "spacecraft" ;
	 nie:keyword "spaceflight" ;
	 nie:keyword "space exploration" ;
	 nie:keyword "missions" ;
	 nie:keyword "satellites" ;
	 nie:keyword "launchers" ;
	 nie:keyword "solar system" ;
	 nie:keyword "Universe" ;
	 nie:keyword "remote sensing" ;
	 nie:keyword "space news" ;
	 nie:keyword "ESA" ;
	 nie:keyword "European" ;
	 nie:keyword "Earth" ;
	 nie:keyword "planets" ;
	 nie:keyword "stars" ;
	 nie:keyword "galaxies" ;
	 nie:keyword "Sun" ;
	 nie:keyword "Moon" ;
	 nie:keyword "astronomy" ;
	 nie:keyword "astrophysics" ;
	 nie:keyword "solar radiation" ;
	 nie:keyword "solar wind" ;
	 nie:keyword "Mars" ;
	 nie:keyword "Jupiter" ;
	 nie:keyword "Venus" ;
	 nie:keyword "Saturn" ;
	 nie:keyword "Neptune" ;
	 nie:keyword "Pluto" ;
	 nie:keyword "Uranus" ;
	 nie:keyword "Herschel" ;
	 nie:keyword "Planck" ;
	 nie:keyword "Eddington" ;
	 nie:keyword "Darwin" ;
	 nie:keyword "SMART" ;
	 nie:keyword "Beagle" ;
	 nie:keyword "Hubble" ;
	 nie:keyword "telescopes" ;
	 nie:keyword "comets" ;
	 nie:keyword "Mercury" ;
	 nie:keyword "observation" ;
	 nie:keyword "LISA" ;
	 nie:keyword "infrared" ;
	 nie:keyword "ultraviolet" ;
	 nie:keyword "x-ray" ;
	 nie:keyword "gamma ray" ;
	 nie:keyword "cosmology" ;
	 nie:keyword "interplanetary" ;
	 nie:keyword "aurora" ;
	 nie:keyword "Cluster" ;
	 nie:keyword "magnetosphere" ;
	 nie:keyword "space weather" ;
	 nie:keyword "life" ;
	 nie:keyword "exoplanets" ;
	 nie:keyword "astrobiology" ;
	 nie:keyword "Huygens" ;
	 nie:keyword "Cassini" ;
	 nie:keyword "BepiColombo" ;
	 nie:keyword "Titan" ;
	 nie:keyword "asteroids" ;
	 nie:keyword "lunar" ;
	 nie:keyword "Rosetta" ;
	 nie:keyword "Giotto" ;
	 nie:keyword "lander" ;
	 nie:keyword "Hipparcos" ;
	 nie:keyword "star-mapping" ;
	 nie:keyword "black holes" ;
	 nie:keyword "atmosphere" ;
	 nie:keyword "nebula" ;
	 nie:keyword "Newton" ;
	 nie:keyword "Integral" ;
	 nie:keyword "XMM" ;
	 nie:keyword "XEUS" ;
	 nie:keyword "Milky Way" ;
	 nie:keyword "hotspots" ;
	 nie:keyword "sunspots" ;
	 nie:keyword "microwaves" ;
	 nie:keyword "Big Bang" ;
	 nie:keyword "relativity" ;
	 nie:keyword "physics" ;
	 nie:keyword "Galileo" ;
	 nie:keyword "STEP" ;
	 nie:keyword "gravity" ;
	 nie:keyword "gravitational waves" ;
	 nie:keyword "microwave background" ;
	 nie:keyword "cosmic rays" ;
	 nie:keyword "orbiter" ;
	 nie:keyword "orbits" ;
	 nie:keyword "SOHO" ;
	 nie:keyword "heliosphere" ;
	 nie:keyword "corona" ;
	 nie:title "A Milky Way twin swept by an ultra-fast X-ray wind / Space Science / Our Activities / ESA" ;
	 


(tracker-miner-apps:1512): Tracker-WARNING **: Couldn't properly parse desktop file 'file:///usr/share/applications/kde4/kgpg.desktop': 'Couldn't load desktop file:'/usr/share/applications/kde4/kgpg.desktop''

(tracker-miner-fs:1576): GLib-CRITICAL **: g_str_has_suffix: assertion 'str != NULL' failed

(tracker-extract:1577): GLib-CRITICAL **: g_str_has_suffix: assertion 'str != NULL' failed

(tracker-extract:1577): GLib-CRITICAL **: g_str_has_suffix: assertion 'str != NULL' failed

(tracker-miner-fs:1576): GLib-CRITICAL **: g_str_has_suffix: assertion 'str != NULL' failed
Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x3400001 ( - Opera)
[0124/083503:ERROR:ticl-message-validator.cc(210)] registration_digest must be non-empty
[0124/083503:ERROR:ticl-message-validator.cc(317)] field registration_summary failed validation in { protocol_version: { version: { major_version: 3 minor_version: 2  }  } client_token: "si8lLsWhaMC/JPP0bRlqIw==" registration_summary: { num_registrations: 0 registration_digest: ""  } server_time_ms: 0  }
[0124/083503:ERROR:ticl-message-validator.cc(359)] field header failed validation in { header: { protocol_version: { version: { major_version: 3 minor_version: 2  }  } client_token: "si8lLsWhaMC/JPP0bRlqIw==" registration_summary: { num_registrations: 0 registration_digest: ""  } server_time_ms: 0  }  }
[0124/083503:ERROR:protocol-handler.cc(143)] Received invalid message: { header: { protocol_version: { version: { major_version: 3 minor_version: 2  }  } client_token: "si8lLsWhaMC/JPP0bRlqIw==" registration_summary: { num_registrations: 0 registration_digest: ""  } server_time_ms: 0  }  }
Entity: line 1: parser error : Document is empty
{
^
Entity: line 1: parser error : Document is empty
{
^
Entity: line 1: parser error : Document is empty
{
^ 


Hope this is of any value to you.

That's the .xsession-errors from your current cinnamon session.  We need the one from your failed plasma session.

Rex:  Where can I find the .xsession-errors from the failed Plasma session?

Same place, ~/.xsession-errors

(though you'll have to try logging in again... that records only the *last* login)

After recent updates I'm also getting errors that after login "could not start ksmserver". I feel this might be related. Killing Xorg and logging in again usually helps then on second or third try.

For posterity, please provide output from:

rpm -q kwin kscreenlocker plasma-workspace

kwin-5.5.5-1.fc23.x86_64
kscreenlocker-5.5.5-2.fc23.x86_64
plasma-workspace-5.5.5-5.fc23.x86_64

Since you mention kscreenlocker: I've lately also experienced a problem where the locker kicked in and threw an error. Upon unlocking Xorg got completely killed and I was at the login-prompt again.

Xorg dying is (most likely) a driver issue

though I suppose ksmserver crashing could lead to similar symptoms, we have no evidence on which to support one theory over the other yet.

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