Unfortunately, I'm still getting the same assertion error, even though I deleted "Firefox.2009-02-08T04-05-56-06-00.dir" - It's recreating the file and then crashing:
AssertionError: QuotedPath: G:\backup of c important stuff.rdb/rdiff-backup-data/increments/portableapps/PortableApps/FirefoxPortable/App/firefox.2009-02-08T04-05-56-06-00.dir
Index: ('portableapps', 'PortableApps', 'FirefoxPortable', 'App', 'firefox.2009-02-08T04-05-56-06-00.dir')
Data: {'uid': 0, 'perms': 438, 'type': 'reg', 'gname': None, 'ctime': 1234324248L, 'devloc': 0, 'uname': None, 'nlink': 0, 'gid': 0, 'mtime': 1171352246L, 'atime': 1234324248L, 'inode': 0L, 'size': 0L}