Backup error 20411

I've been having a problem with FMS 12 backup schedules for the last few days, and it's giving an error code - 20411 - that I haven't been able to find documentation on.

I currently have Hourly, Daily, and Weekly schedules running; Hourly to an internal drive, Daily and Weekly to an external USB drive. The problem is occurring with all three backup schedules, so I'm inclined to think it's not a problem with either a specific schedule, or with the backup destination. The database is hosted on a RAID 1 mirrored drive, and one of the drives recently failed; however, that doesn't seem to have affected the operation of the main database.

Essentially, the backup starts, gets partway through, and then aborts. I get the following sequence of error messages:

"Backup aborted by user or due to error; some incomplete files may be left in destination (20411)"

"Schedule "Hourly" aborted; unexpected error. (20411)"

Looking at the backup destination folder, it appears to have copied all but the last two .fmp12 files successfully; those two files exist in the destination, but only as 12KB placeholders. One thing about those two files that is different is that both use external container storage, while the other files do not; I'm not sure if that makes a difference.

I've tried stopping and re-starting the FMS service, and also tried rebooting the entire machine, with no success.

The machine is Windows Server 2008 R2 Standard, running on a Dell R610 server with 12 GB of RAM.

Anyone know first, what error 20411 is; and second, anyone have any idea what the underlying problem might be?

Many thanks!