Time machine "backup volume couldnot be mounted"

I have a time machine backup disc connected to my airport base station. I've been using time machine to back up both a mac book and a mini. Recently the mini will generate a error message "Time machine error The back up disc image could not be mounted" and the back up will fail. I've verified that I can get to the backup drive from the Mac mini and open files. I also consulted the apple support online for this issue. I've both verified the network volumes password in the keychain access utility and I have unselected and reselected the drive in Time machine set up. The Mac book has no issues so I'm stumped as to my next steps. Any ideas out there? I'm using OS X 10.5.8 on both computers.