Plasma lock screen stuck

[b-misc@gmx.ch]

Hi all,

I've just (dist-)upgraded a laptop from jessie to stretch and now there's one 
problem left concerning KDE / Plasma:

After closing the lid and re-opening it, everything seems stuck, i.e. if I 
enter the password nothing is shown in the password field, and if I click on 
Change User, nothing happens as well. But the machine is still alive and I can 
change to a text console and after restarting sddm using systemd there I can 
re-login graphically again.

Any ideas what the problem might be?

Thank you, kind regards,
Bernd