I haven't noticed the phenomenon you mention, Stephan. But in recent days, I can often no longer view and panels left or right on my selected image, not in develop or print or other modules. The screen is just black. The panels show up properly only in the library module. My fix is also to restart LR6 (not CC), but after a few times a day, that becomes tedious. I'm using all the latest software with el Capitan too. This is the first problem I've encountered with LR since version 1. I'll be tracking your progress - maybe there's a connection to my problem.