Time Capsule is already in use

My Time Capsule was running perfectly saving backups off of 2 different Macs. This week I received a message that one of the macs cant back up because its in use. The other computer is backing up perfectly. As far as I can see this is not running in the background and the other computer has not changed, its still backing up to the correct file, As far as I can see...

The computer is a iMac with 10.8.2. We have plenty space on the time capsule. the error is:

"The backup disk image :/Volumes/data/(back kup name).iMac.sparsebundle" is already in use"

Does anyone know how to correct this?