OfflineIMAP 6.5.6 consistently dying on an enormous label on Gmail

[Rich]

Hi all,
I'm running a self-installed 6.5.6 on Debian Wheezy, and have recently
resumed syncing my mail from Gmail after a long period of accidentally
forgetting to start the daemon.

So it's been catching up on a _lot_ of (somewhat inane) messages, but
the process dies periodically with messages like:
Thread 'Copy message from Remote:PhA/1 pha!bb' terminated with exception:
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/offlineimap/threadutil.py",
line 158, in run
    Thread.run(self)
  File "/usr/lib/python2.7/threading.py", line 505, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/local/lib/python2.7/dist-packages/offlineimap/folder/Gmail.py",
line 277, in copymessageto
    super(GmailFolder, self).copymessageto(uid, dstfolder,
statusfolder, register)
  File "/usr/local/lib/python2.7/dist-packages/offlineimap/folder/Base.py",
line 581, in copymessageto
    message = self.getmessage(uid)
  File "/usr/local/lib/python2.7/dist-packages/offlineimap/folder/Gmail.py",
line 77, in getmessage
    data = self._fetch_from_imap(imapobj, str(uid), 2)
  File "/usr/local/lib/python2.7/dist-packages/offlineimap/folder/IMAP.py",
line 659, in _fetch_from_imap
    imapobj.select(self.getfullname(), readonly = True)
  File "/usr/local/lib/python2.7/dist-packages/offlineimap/imaplibutil.py",
line 66, in select
    raise OfflineImapError(errstr, severity)
OfflineImapError: Error SELECTing mailbox 'PhA/1 pha!bb', server reply:
('NO', ['System error (Failure)'])

(PhA is a label, 1 pha!bb is a label under that label).

It does manage to retrieve a few messages from that label usually
before dying...

I can make the debug logfile available to a developer, but since it
contains a number of personal details about how I have laid out my
mailbox (among other things), I'd rather not share it with the entire
mailing list...

Let me know if there's anything other than the debug log that would be
useful to know, or contact me off-list if you're a developer and would
like access to said log.

Thanks,
- Rich