KDE no longer starts after login

[mls at xlist.pw]

On Sunday 30 March 2014 15:39:08 Ralf Mardorf wrote:
> That's bad. I experience issues when using RT patched kernels > 3.8.x.
> My machine will lock and there are no helpful log messages.
> 
> The kernel-rt folks ask me to connect another computer by parallel port
> to see what's happening, the Arch audio people recommended to remove
> cards, to see if the kernel would work after a card is removed. No
> options for me right now.
> 
> However, when using those kernel-rt's > 3.8.x sometimes the machine
> locks when doing the login. Your issue might be caused by something
> completely different, but for troubleshooting you might want to test an
> older kernel.
> 
> I suspect you're using the default kernels, nothing special as e.g. a RT
> patched kernel?

I use just the default kernel:
#uname -a
Linux MYHOST 3.11.0-18-generic #32-Ubuntu SMP Tue Feb 18 21:11:14 UTC 2014 
x86_64 x86_64 x86_64 GNU/Linux

My problem is not that the machine hangs. On the text console Alt-F2-F6 I can 
work without any problem. Only the graphical console Alt-F7 hangs after logon.