Backup creating invalid database file

I've got a maintenance plan running that does a validation and then a backup using

backup database directory 'P:\\GKBackups' wait before start transaction log rename;

There seems to be something wrong with the database or something, because the backup file is 'not a valid database file', and it seems the event for the maintenance plan never finished (connections were still disabled; never got email).

Any thoughts?

SQLA 10.0.1.3835

UPDATE: I did some logging, but I got NOTHING. The backup just never finishes. It never throws an exception either. I have absolutely nothing to go on. The strange thing is, though, that dbbackup.exe will work, just the backup database command will not.