The error message was always "Invalid course module id", sometimes when trying to import courses or quizzes from old courses, sometimes when accessing quizzes, sometimes when accessing the grader.
Affected courses seem to take a long time when creating backup files.
This just happened 3 times in around 2 weeks at our site, so it does not seem to be widespread. However, I was not able to reproduce it and thus it makes me feel a little bit uncomfortable, like I might be sitting on a time bomb of a lot of broken courses.
Did anyone else encounter this or similar problems? Is this a bug that is known or already fixed?