Why my database mirror break down after changing file group settings from RESTRICTED_USER to MULTI_USER?
We found the issue. It is a bug in SQL Server. When we set READ_WRITE the command is not transfeered properly into the mirror DB. When script start change partitions on the mirror server an error occured. After that the syncronization is ruined and the DB on the mirror is locked (in suspended state).

We fix the issue by updating SQL Server to the latest version (our initial version was wiout SP).