Just to clarify, this statement is incorrect "A while ago our arcseve db started crashing each night. " Your ARCserve database has not crashed.
Check to see if there are tapes in the Scratch Set. If there are none then it is correct that it would ask for a blank tape. If there are tapes in the Scratch Set then there is a good chance there is database corruption.
Which DB Agent is being used?
Is the database running on the same server as ARCserve or a different one?
Have you verified the integrity of the database, and checked it's logs for errors.