Finally it works
Thanks for all the help
Here a list of things to be aware of:
1. Be logged on as the user under whom the backupExec services run
2. Delete all Files in LU-Downloadsfolder
3. Cut the connection to all MMS if you update on CAS an deinstall Managed Server Options if you update a MMS
3a. for reinstall the managed server options be shure to use User with administrator-rights on CAS
4. Re-Attach BEDB database if there is still trouble (Backup Database before changing anything)
And thanks again for the help