I recently updated to El Capitan 10.11.2 of OSX and within the same day after the OS tried to do a backup on my Time Capsule at home the backup does not work anymore. I have already posted the problem on apple support communities. Backups from other versions of OSX run perfectly correct against the same Time Capsule so don’t let you lure into support procedures like do a clean install….this is a problem with the OS. It seems that the sparesbundle gets corrupted and therefore can only be mounted in readonly mode because there are no more mountable filesystems. Link to the Support Community entry.