In case anyone else also encounters this problem, I've found that it relates to the new Apple 7.5.2 firmware for the Time Capsule. After re-installing AirPortUtility551 (firmware 7.5.1), I am able to a) delete all those pesky temporary rsync files from the Time Capsule without having to resort to "erase" and b) use Espionage to perform backups to my Time Capsule again.
cheers,
Niel
cheers,
Niel