Waiting for Backup Set - but it's not in use!

[acovington 0]

Using Retrospect 7.5.324 Multi Server, and a job I've used for a couple of years now won't execute, instead it's "Waiting for" it's (disk) Backup Set! The Events tab tells me it can't open the Catalog File for that backup set (-843). I've even rebooted the server, but this persists! No other Retrospect jobs are running. How can I find the source of the problem, and how do I "break" the lock? I've searched the forum but can't find a good answer. Any help is appreciated, thank you.