Time Machine: Error -1

I've been getting this message for the past day or so: "The backup disk image '[…] name.sparsebundle' could not be accessed (error -1)." Any explanations? (Time Machine Error) Mac OS X 10.6.8

I seem to remember that there was an AirPort Utility update recently, but I can't remember whether there was a firmware update included in that as well. I haven't seen this error message before, so I'm not sure whether that could be the culprit.

Ideas? (I'm the kind who likes to figure out what the problem actually *is* rather than just reboot everything a few times and hope for the best.)

-- Tim