Time Machine Could Not Complete The Backup

Hello all,

Beginning this morning, my MBP has been unable to back up to Time Machine, with the error being "An error occurred while creating the backup folder."

Setup:
An external USB drive is attached to my iMac and shared over the network. The iMac, my MacBook Air, and my MacBook Pro all backup to this drive.

The iMac has no issues backing up.

The Air has no issues backing up. I even tested it with an user-initiated backup after the MBP started having problems, and the backup completed without trouble.

The MBP has been backing up for three months to this drive without issue. The problem began sometime in the last 24 hours.

Already Tried:
http://Pondini.org/TM/A5.html
http://Pondini.org/TM/A4.html

Neither of the above solution has any effect.

Rebooting the MBP has had no effect.

That is all the information I have for now. Any ideas?

Many thanks.