Please reassign appropriately, I have no idea what the proper package
would be, sorry.

I usually don't use plasma (my wife does), so I never configured it.
So most likely I have pretty much the initial setup.

By accident I just logged in using Plasma (X11) and wanted to log out.

First, its very hidden how to log out. Often, we are working in
successsion, so some time ago we searched and found out how to log
out. It is not very good UI, its quit hidden, but this is not the bug
report.

So I went the arrow and "Verlassen" (Leave) appeared. However, I could
not select it. When I moved the mouse in the direction of this word,
it vanished. Also the old Unix trick (ctrg + alt + backspace) did not
work either.

I had to open a root shell and kill the process startplasma.