2 questions:

What plasma theme are you using?

Possible to try running plasmashell by hand from this "black screen" session (via konsole or krunner)?  If it crashes, we'll need a backtrace.

Re-opening upstream report of similar issue I hit while testing (but I thought I'd fixed it in plasma-workspace-5.8.6-2 packaging),
https://bugs.kde.org/show_bug.cgi?id=376847

I get the issue too :( Theme is Breeze Dark. And I cannot start anything with krunner. Had to downgrade to get a working system again.

Logs show no obvious issues, just some messages I get with 5.8.5 too:Feb 25 17:23:15 bud plasmashell[1358]: QXcbConnection: XCB error: 2 (BadValue), sequence: 22016, resource id: 113246224, major code: 141 (Unknown), minor code: 3

Rex, the issue happens with both the Fedora Breeze and regular Breeze themes.  I tried using krunner, but got the same result as Christian... was able to bring up the krunner command line, but when i entered a command, nothing happened.

breeze-icon-theme-5.31.0-1.fc25 extra-cmake-modules-5.31.0-3.fc25 kf5-5.31.0-1.fc25 kf5-attica-5.31.0-1.fc25 kf5-baloo-5.31.0-1.fc25 kf5-bluez-qt-5.31.0-1.fc25 kf5-frameworkintegration-5.31.0-1.fc25 kf5-kactivities-5.31.0-1.fc25 kf5-kactivities-stats-5.31.0-1.fc25 kf5-kapidox-5.31.0-1.fc25 kf5-karchive-5.31.0-1.fc25 kf5-kauth-5.31.0-1.fc25 kf5-kbookmarks-5.31.0-1.fc25 kf5-kcmutils-5.31.0-1.fc25 kf5-kcodecs-5.31.0-1.fc25 kf5-kcompletion-5.31.0-1.fc25 kf5-kconfig-5.31.0-1.fc25 kf5-kconfigwidgets-5.31.0-1.fc25 kf5-kcoreaddons-5.31.0-1.fc25 kf5-kcrash-5.31.0-1.fc25 kf5-kdbusaddons-5.31.0-1.fc25 kf5-kdeclarative-5.31.0-1.fc25 kf5-kded-5.31.0-1.fc25 kf5-kdelibs4support-5.31.0-1.fc25 kf5-kdesignerplugin-5.31.0-1.fc25 kf5-kdesu-5.31.0-1.fc25 kf5-kdewebkit-5.31.0-1.fc25 kf5-kdnssd-5.31.0-1.fc25 kf5-kdoctools-5.31.0-1.fc25 kf5-kemoticons-5.31.0-1.fc25 kf5-kfilemetadata-5.31.0-1.fc25 kf5-kglobalaccel-5.31.0-1.fc25 kf5-kguiaddons-5.31.0-1.fc25 kf5-khtml-5.31.0-1.fc25 kf5-ki18n-5.31.0-1.fc25 kf5-kiconthemes-5.31.0-1.fc25 kf5-kidletime-5.31.0-1.fc25 kf5-kimageformats-5.31.0-1.fc25 kf5-kinit-5.31.0-1.fc25 kf5-kio-5.31.0-1.fc25 kf5-kitemmodels-5.31.0-1.fc25 kf5-kitemviews-5.31.0-1.fc25 kf5-kjobwidgets-5.31.0-1.fc25 kf5-kjs-5.31.0-1.fc25 kf5-kjsembed-5.31.0-1.fc25 kf5-kmediaplayer-5.31.0-1.fc25 kf5-knewstuff-5.31.0-1.fc25 kf5-knotifications-5.31.0-1.fc25 kf5-knotifyconfig-5.31.0-1.fc25 kf5-kpackage-5.31.0-1.fc25 kf5-kparts-5.31.0-1.fc25 kf5-kpeople-5.31.0-1.fc25 kf5-kplotting-5.31.0-1.fc25 kf5-kpty-5.31.0-1.fc25 kf5-kross-5.31.0-1.fc25 kf5-krunner-5.31.0-1.fc25 kf5-kservice-5.31.0-1.fc25 kf5-ktexteditor-5.31.0-2.fc25 kf5-ktextwidgets-5.31.0-1.fc25 kf5-kunitconversion-5.31.0-1.fc25 kf5-kwallet-5.31.0-1.fc25 kf5-kwayland-5.31.0-1.fc25 kf5-kwidgetsaddons-5.31.0-1.fc25 kf5-kwindowsystem-5.31.0-1.fc25 kf5-kxmlgui-5.31.0-1.fc25 kf5-kxmlrpcclient-5.31.0-1.fc25 kf5-modemmanager-qt-5.31.0-1.fc25 kf5-networkmanager-qt-5.31.0-1.fc25 kf5-plasma-5.31.0-1.fc25 kf5-solid-5.31.0-1.fc25 kf5-sonnet-5.31.0-1.fc25 kf5-syntax-highlighting-5.31.0-1.fc25 kf5-threadweaver-5.31.0-1.fc25 oxygen-icon-theme-5.31.0-1.fc25 has been submitted as an update to Fedora 25. https://bodhi.fedoraproject.org/updates/FEDORA-2017-0f6aa187ff

Plasma-5.8.6 was built against kf5-5.31.0 so essentially depends on it.

Since kf5 is so big, I submitted it seprately,
https://bodhi.fedoraproject.org/updates/FEDORA-2017-0f6aa187ff
to bodhi prior to submitting this one. No idea why that one didn't get pushed to -testing yet. :(

breeze-icon-theme-5.31.0-1.fc25, extra-cmake-modules-5.31.0-3.fc25, kf5-5.31.0-1.fc25, kf5-attica-5.31.0-1.fc25, kf5-baloo-5.31.0-1.fc25, kf5-bluez-qt-5.31.0-1.fc25, kf5-frameworkintegration-5.31.0-1.fc25, kf5-kactivities-5.31.0-1.fc25, kf5-kactivities-stats-5.31.0-1.fc25, kf5-kapidox-5.31.0-1.fc25, kf5-karchive-5.31.0-1.fc25, kf5-kauth-5.31.0-1.fc25, kf5-kbookmarks-5.31.0-1.fc25, kf5-kcmutils-5.31.0-1.fc25, kf5-kcodecs-5.31.0-1.fc25, kf5-kcompletion-5.31.0-1.fc25, kf5-kconfig-5.31.0-1.fc25, kf5-kconfigwidgets-5.31.0-1.fc25, kf5-kcoreaddons-5.31.0-1.fc25, kf5-kcrash-5.31.0-1.fc25, kf5-kdbusaddons-5.31.0-1.fc25, kf5-kdeclarative-5.31.0-1.fc25, kf5-kded-5.31.0-1.fc25, kf5-kdelibs4support-5.31.0-1.fc25, kf5-kdesignerplugin-5.31.0-1.fc25, kf5-kdesu-5.31.0-1.fc25, kf5-kdewebkit-5.31.0-1.fc25, kf5-kdnssd-5.31.0-1.fc25, kf5-kdoctools-5.31.0-1.fc25, kf5-kemoticons-5.31.0-1.fc25, kf5-kfilemetadata-5.31.0-1.fc25, kf5-kglobalaccel-5.31.0-1.fc25, kf5-kguiaddons-5.31.0-1.fc25, kf5-khtml-5.31.0-1.fc25, kf5-ki18n-5.31.0-1.fc25, kf5-kiconthemes-5.31.0-1.fc25, kf5-kidletime-5.31.0-1.fc25, kf5-kimageformats-5.31.0-1.fc25, kf5-kinit-5.31.0-1.fc25, kf5-kio-5.31.0-1.fc25, kf5-kitemmodels-5.31.0-1.fc25, kf5-kitemviews-5.31.0-1.fc25, kf5-kjobwidgets-5.31.0-1.fc25, kf5-kjs-5.31.0-1.fc25, kf5-kjsembed-5.31.0-1.fc25, kf5-kmediaplayer-5.31.0-1.fc25, kf5-knewstuff-5.31.0-1.fc25, kf5-knotifications-5.31.0-1.fc25, kf5-knotifyconfig-5.31.0-1.fc25, kf5-kpackage-5.31.0-1.fc25, kf5-kparts-5.31.0-1.fc25, kf5-kpeople-5.31.0-1.fc25, kf5-kplotting-5.31.0-1.fc25, kf5-kpty-5.31.0-1.fc25, kf5-kross-5.31.0-1.fc25, kf5-krunner-5.31.0-1.fc25, kf5-kservice-5.31.0-1.fc25, kf5-ktexteditor-5.31.0-2.fc25, kf5-ktextwidgets-5.31.0-1.fc25, kf5-kunitconversion-5.31.0-1.fc25, kf5-kwallet-5.31.0-1.fc25, kf5-kwayland-5.31.0-1.fc25, kf5-kwidgetsaddons-5.31.0-1.fc25, kf5-kwindowsystem-5.31.0-1.fc25, kf5-kxmlgui-5.31.0-1.fc25, kf5-kxmlrpcclient-5.31.0-1.fc25, kf5-modemmanager-qt-5.31.0-1.fc25, kf5-networkmanager-qt-5.31.0-1.fc25, kf5-plasma-5.31.0-1.fc25, kf5-solid-5.31.0-1.fc25, kf5-sonnet-5.31.0-1.fc25, kf5-syntax-highlighting-5.31.0-1.fc25, kf5-threadweaver-5.31.0-1.fc25, oxygen-icon-theme-5.31.0-1.fc25 has been pushed to the Fedora 25 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-0f6aa187ff

new plasma-desktop/plasma-workspace builds add a versioned dependency on kf5-plasma (from kde frameworks 5.31.0)

After update new builds of kf5* packages system login successfully. @Rex thanks for quick fix.

breeze-icon-theme-5.31.0-1.fc25, extra-cmake-modules-5.31.0-3.fc25, kf5-5.31.0-1.fc25, kf5-attica-5.31.0-1.fc25, kf5-baloo-5.31.0-1.fc25, kf5-bluez-qt-5.31.0-1.fc25, kf5-frameworkintegration-5.31.0-1.fc25, kf5-kactivities-5.31.0-1.fc25, kf5-kactivities-stats-5.31.0-1.fc25, kf5-kapidox-5.31.0-1.fc25, kf5-karchive-5.31.0-1.fc25, kf5-kauth-5.31.0-1.fc25, kf5-kbookmarks-5.31.0-1.fc25, kf5-kcmutils-5.31.0-1.fc25, kf5-kcodecs-5.31.0-1.fc25, kf5-kcompletion-5.31.0-1.fc25, kf5-kconfig-5.31.0-1.fc25, kf5-kconfigwidgets-5.31.0-1.fc25, kf5-kcoreaddons-5.31.0-1.fc25, kf5-kcrash-5.31.0-1.fc25, kf5-kdbusaddons-5.31.0-1.fc25, kf5-kdeclarative-5.31.0-1.fc25, kf5-kded-5.31.0-1.fc25, kf5-kdelibs4support-5.31.0-1.fc25, kf5-kdesignerplugin-5.31.0-1.fc25, kf5-kdesu-5.31.0-1.fc25, kf5-kdewebkit-5.31.0-1.fc25, kf5-kdnssd-5.31.0-1.fc25, kf5-kdoctools-5.31.0-1.fc25, kf5-kemoticons-5.31.0-1.fc25, kf5-kfilemetadata-5.31.0-1.fc25, kf5-kglobalaccel-5.31.0-1.fc25, kf5-kguiaddons-5.31.0-1.fc25, kf5-khtml-5.31.0-1.fc25, kf5-ki18n-5.31.0-1.fc25, kf5-kiconthemes-5.31.0-1.fc25, kf5-kidletime-5.31.0-1.fc25, kf5-kimageformats-5.31.0-1.fc25, kf5-kinit-5.31.0-1.fc25, kf5-kio-5.31.0-1.fc25, kf5-kitemmodels-5.31.0-1.fc25, kf5-kitemviews-5.31.0-1.fc25, kf5-kjobwidgets-5.31.0-1.fc25, kf5-kjs-5.31.0-1.fc25, kf5-kjsembed-5.31.0-1.fc25, kf5-kmediaplayer-5.31.0-1.fc25, kf5-knewstuff-5.31.0-1.fc25, kf5-knotifications-5.31.0-1.fc25, kf5-knotifyconfig-5.31.0-1.fc25, kf5-kpackage-5.31.0-1.fc25, kf5-kparts-5.31.0-1.fc25, kf5-kpeople-5.31.0-1.fc25, kf5-kplotting-5.31.0-1.fc25, kf5-kpty-5.31.0-1.fc25, kf5-kross-5.31.0-1.fc25, kf5-krunner-5.31.0-1.fc25, kf5-kservice-5.31.0-1.fc25, kf5-ktexteditor-5.31.0-2.fc25, kf5-ktextwidgets-5.31.0-1.fc25, kf5-kunitconversion-5.31.0-1.fc25, kf5-kwallet-5.31.0-1.fc25, kf5-kwayland-5.31.0-1.fc25, kf5-kwidgetsaddons-5.31.0-1.fc25, kf5-kwindowsystem-5.31.0-1.fc25, kf5-kxmlgui-5.31.0-1.fc25, kf5-kxmlrpcclient-5.31.0-1.fc25, kf5-modemmanager-qt-5.31.0-1.fc25, kf5-networkmanager-qt-5.31.0-1.fc25, kf5-plasma-5.31.0-1.fc25, kf5-solid-5.31.0-1.fc25, kf5-sonnet-5.31.0-1.fc25, kf5-syntax-highlighting-5.31.0-1.fc25, kf5-threadweaver-5.31.0-1.fc25, oxygen-icon-theme-5.31.0-1.fc25 has been pushed to the Fedora 25 stable repository. If problems still persist, please make note of it in this bug report.