Thanks for the fast answer.
I did a clean install with the new version. Then I noticed you can set it to include checking for cmp updates too, I did that and then the launcher chrashed every time after checking for updates.
After that, I installed the now old 2.55 Launcher from the website again, but didn“t change anything else. I could play on the public server, I could play the campaign and all coop stuff too. Only Motovskiy 32 crashed while loading in the objects stage, so I concluded it must be a problem with the objects in that version of the map.
I mean, how could an older launcher cause a problem with only one layer of a map?