[root@shutdown ~]# mount -t cifs -o user=nobody,guest //mir-qs/on3 /mnt/tmp/
mount error 13 = Permission denied
Refer to the mount.cifs(8) manual page (e.g.man mount.cifs)

Error message on the server is

make_connection: connection to on3 denied due to security descriptor.

Googling around led me to the belief that someone fooled around with
srvmgr.exe from a windows machine and that I should remove
/var/cache/samba/share_info.tdb and restart samba. Which doesn't work.

This is my smb.conf (well, only the most important parts):

[global]
    workgroup = FOOBAR
    server string = My Server
    map to guest = Bad User
    preferred master = No
    local master = No
    domain master = No
    dns proxy = No

[on3]
    comment = Audio-Video-Imports
    path = /local/mir/import/on3
    force group = users
    read only = No
    create mask = 0664
    directory mask = 0775
    guest ok = Yes

This is my smbusers:
root = administrator admin
nobody = guest pcguest smbguest

Now if I take out the "force group = users" everything works as
expected. Except that I cannot write in this share - nobody isn't in the
group users.

smbclient works, mounting those shares from Windows also works.

Mounting does not work from CentOS 3, 4, 5 or OS X (command line).