It was only a guess for now and only based on the xsession errors I found logged. Was there another package checked-in for 20160523 related to the graphics stack? The bad thing for hunting down: it only occurs quite rare and my feeling is that it may be more often when cpu is working heavily. The good thing for me: it only occurs quite rare :-) What is the best way to downgrade the multiple libxcb-packages in one run?