Hello again
The problem appeared again after a short moment. User overrides did´nt work - not either assignments for the teacher but the students.
I was shouting yes to early - It was more likely to be the same problem as Sergio had.
https://tracker.moodle.org/browse/MDL-60987
We reverted the php-version from 7.1 to 7.0 and suudenly it disappeared!