I would still check out your keys. On your backuppc server, su to your backuppc user and run:
ssh root@one-of-your-servers-to-be-backed-up whoami
and you should not get anything returned but root. You might get prompted to re-accept the SSH key - accept it and run again and you should get just root again.
Or, if you get a different error, fix that and you should be set.