Unfortunately an Exchange reboot didn't resolve this issue, I've moved the faulting database backups over to the 2nd data center and will back up the passive copy.
After the reboot of Exchange, I'm back at square one:
An attempt to prepare the database 'MailDB' for backup failed because the database is already in the process of being backed up. The error code is -2403. (Note that if a backup was recently aborted, then it may take several minutes for the system to detect the aborted backup and initiate backup cleanup procedures, so this message may be generated if an attempt was made to backup a database before a previous backup attempt had fully terminated.)
Exchange VSS Writer (instance 542dc4c9-c31d-4316-8ef8-75a4afe458c1:44) failed with error code -2403 when preparing the database engine for backup of database 'MailDB'
