OnStream Drive ejects tape during Backup

I recently inherrited a Win2k network.
The first few weeks all was well, but suddenly the OnStream backup-tape seems to be ejected during the backup (with ntbackup). The result is that in the morning ntbackup is still displaying all kinds of errormessages and I have to kill the program.
The log-file looks like it’s just cut in half and ends in the middle of an entry.
There is no helpful event in the system- or applicationlog, except that I terminated the backup program.