After working with a quiz I now have too many instances of the Navigation and Administration blocks on the quiz preview and/or front page.
In one quiz I have 4 instances of each of the two blocks on the preview page, in another quiz I have 2 instances of each of them on the front page - in Firefox as well as in Chromium. Students are only affected on the quiz front page.
Unfortunately I can not tell when or under which circumstances they got duplicated. After running a 'git pull' on the server and after purging all cache the issue remains. I have constantly switched themes between boost and clean (QA testing), but I can not tell if that is related to the issue. I will let you know if I get closer to the problem.