A few days ago, Time Machine on my Mac reported an error: ‘Sparsebundle already in use’. Manual unmount of the Time Capsule didn't help, nor a reboot of my machine. Removing and re-selecting my Time Capsule from the Time Machine Preferences window didn't help either.
After some research, I found the answer was pretty straightforward, though not obvious. As the error message hints, the problem is that the disk mount utility believes that there is already a user connected to the disk. To solve that, you can use a handy function present in Airport Utility 5.6 (but not in Airport Utility 6.1), named ‘disconnect all users’:
If you don't have the Airport Utility 5.6, you can either download it from Apple's KB (if you're on Mountain Lion you'll also need the Pacifist app to extract the app from the package) or just use your Airport Utility 6.1 to reboot the device.