10.11.2: QuickLook and thumbnails crashing finder
I was able to solve it:

sudo /usr/libexec/repair_packages --repair --standard-pkgs /

did repair the access rights of the folder /private/tmp. I suspect that Adobe Creative Cloud has altered the rights. 

There have been some log lines in the console about quicklookd being not able to scan /, so that is another symptom of that problem.