I'm testing the upgrade on our testing system with courses (database) as a copy from the prod. system, while I got the users to use the prod. system as we made a release to the newer moodle version after checking out the main things. Now an elder course turned out to make the mentioned problem on the testing system, but works on the prod. system which is quite weird!
This is the history. I can't run the tex debug routine on the testing system but on the prod. system, that's weird too! So I don't have any results for the testing system otherwise I wouldn't have posted these.
I recognized that my discussion point I posted yesterday has a not up-to-date version in this forum where you can't see the functions I tested. Don't know why this old version is in the forum?