Greg Madden wrote:

> I have the same issue, and I have never used the "screen lock" option in
> the start menu.
> 
> Mine is caused bythe Nvidia driver, from vendor, for my Quadro card,
> does not play well with TDE.
> 
> You don't mention video card info or drivers, could help.
> 
> ps, my solution was to rename the executable responsible:
> /opt/trinity/bin/kdesktop_lock to kdesktop_lock.bk

I think this is a problem with kdesktop_lock in general, perhaps also
related with different themes.
The best would be to file a bug https://bugs.trinitydesktop.org, so that we
do not forget.
there are few bugs against kdesktop_lock already

regards