OK, I installed the update, emptied the Autoptimize-cache and retried: Same result. Then I deactivated the complete plugin and everything worked like a charm. To be sure, I re-activated it and, as expected, the problem reappeared. As I would like to keep the plug-in, I had to go through the options and finally found that deactivating the option “optimize CSS code” finally did the trick. Thank you for your help!