I solved the problem...
1) the deinstallation of feedback: I had to delete also the feedback folder in blocks, otherwise I could not see anything on my moodle platform
2) I had to change Session Handling to "Use database for session information"
3) I could then intall Questionaire without any problems
4) feedback again made problems, but after I manually installed the tables in phpMyAdmin, it now works... (no idea, why the tables did not install automatically...)