My retrospect was having the same error with backing up storage groups. After speaking with tech support (we have a support contract) it was suggested that the exchange server thinks there is still a backup being performed and will not allow the scripted one to start. I was asked to reboot the exchange server and that fixed the problem. Maybe this will work for you guys.
I have the same version and hotfix with the same issue. We have two storage groups backed up weekly using a script.. and the first storage group is receiving the same error that you are having. Robin: Checked passwords, nothing seems to be touched. No responses yet, wondering if anyone is still experiencing this issue. Might have to call Retrospect if nothing is posted soon