Hi PsyDye,
thanks a lot for the reply.
I tried again the normal way: Sidebar, click on Playlist, little pause, click again on playlist … No renaming enabled. And by the way: Same goes for renaming a playlist folder.
Then, I tried a another way: click on playlist in sidebar, click within the playlist window on the playlist name at the top … That worked. I can rename the name there and after that it changes in the sidebar, too.
So, I have a solution ! But it seems, this is still a bug, as it makes no sense for me, that the functionality of renaming directly within the sidebar, should be somehow taken away by Apple.
Thank you again for your help.
Have a fruitful day.