"binascii.Error: Incorrect padding"

[ael]

I have a problem with a the imap server from a major isp in the uk.
It used to work fine with offlineimap.
Now I get:
--------------------------

$ offlineimap --info -a bt
OfflineIMAP 6.6.1
  Licensed under the GNU GPL v2 or any later version (with an OpenSSL exception)
Remote repository 'btserv': type 'IMAP'
Host: mail.btinternet.com Port: None SSL: True
Establishing connection to mail.btinternet.com:993
Traceback (most recent call last):
  File "/usr/bin/offlineimap", line 23, in <module>
    oi.run()
  File "/usr/share/offlineimap/offlineimap/init.py", line 52, in run
    self.__serverdiagnostics(options)
  File "/usr/share/offlineimap/offlineimap/init.py", line 429, in __serverdiagnostics
    account.serverdiagnostics()
  File "/usr/share/offlineimap/offlineimap/accounts.py", line 197, in serverdiagnostics
    self.ui.serverdiagnostics(remote_repo, 'Remote')
  File "/usr/share/offlineimap/offlineimap/ui/UIBase.py", line 436, in serverdiagnostics
    conn = repository.imapserver.acquireconnection()
  File "/usr/share/offlineimap/offlineimap/imapserver.py", line 495, in acquireconnection
    self.__authn_helper(imapobj)
  File "/usr/share/offlineimap/offlineimap/imapserver.py", line 397, in __authn_helper
    if func(imapobj):
  File "/usr/share/offlineimap/offlineimap/imapserver.py", line 321, in __authn_plain
    imapobj.authenticate('PLAIN', self.__plainhandler)
  File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 705, in authenticate
    typ, dat = self._simple_command('AUTHENTICATE', mechanism.upper())
  File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 1695, in _simple_command
    return self._command_complete(self._command(name, *args), kw)
  File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 1421, in _command
    literal = literator(data, rqb)
  File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 2286, in process
    ret = self.mech(self.decode(data))
  File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 2316, in decode
    return binascii.a2b_base64(inp)
binascii.Error: Incorrect padding

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Before I start digging further, is this a well-known problem?

ael