Sorry don't speak German but from Google translate that sounds like you are getting a please insert overwriteable media message when running backup jobs.
This is likely to be
1) You are backing up more data than you used to and it now needs more tape capacity as it asks for a second tape (this will show as some data backs up and then the error)
2) You are appending to the tape when you should be overwriting so it does not have enough space (this will also show as some data written and then the error)
3) There is no overwriteble media mounted in the library/tape device at the time the job starts (this will error near the start of the job)
Option 1 can only be fixed by reducing the data or by investigating in larger tape technologies
Options 2 and 3 are controlled by the media settings in the backup jobs, the media set propeties and the overall Media Overwrite Settings in the Media Management configuration for the server. Just be aware that if you are running into option 3 when you think you shoudln't be then it might be because you have not realized that he counter for the overwrite protection starts when the last job to use the tape finishes so if you have a 4 week rotaion on tapes you can't set a 4 week overwrite, you have to set 3 weeks, 6 days (in days) instead to allow it to be overwriteable for the start of the next job.
