KDE no longer starts after login

[mls at xlist.pw]

Hi,

on a Kubuntu 13.10 64-bit system suddenly KDE does not start up anymore for 
all users after login. lightdm shows the login screen, after user login only 
the lightdm-kde-greeter background is shown, but no KDE startup happens.

In lightdm.log I see
...
DEBUG: Session pid=1719: Started with service 'lightdm', username 'MYUSER'
DEBUG: Session pid=1719: Got 1 message(s) from PAM
DEBUG: Session pid=1772: Prompt greeter with 1 message(s)
DEBUG: Session pid=1772: Continue authentication
DEBUG: Session pid=1719: Authentication complete with return value 0: Success
DEBUG: Session pid=1772: Authenticate result for user MYUSER: Success
DEBUG: Session pid=1772: User MYUSER authorized

and the log stops.

On another PC with 13.10 the lines above are followed by

DEBUG: Session pid=1772: Greeter requests session kde-plasma
DEBUG: Writing /home/MYUSER/.dmrc
DEBUG: Seat: Stopping greeter; display server will be re-used for user session
...

Can someone give me a hint how I can further analyse what is going wrong when 
starting the kde-plasma session.

Thanks & regards,

mls