Hmm. Did you happen to change the password at any time (and so get asked if you wanted to back up everything again next time)? I've just found, and fixed, a problem with that which would have resulted in the behaviour you describe (so thanks for the bug report :-)

Otherwise, it could be that there isn't anything else big enough to fit into the remaining space - PeerBackup makes sure there is enough space left before it tries to back up each file. When that happens, the files which wouldn't fit will be listed in the backup as having failed because there isn't enough space available.