CookieStorage for contrib.messages crashes after upgrade to django 3.2

[Jan Pieter Waagmeester]

After upgrading to django 3.2, a prevously stored cookie for contrib.messages crashes in

​https://github.com/django/django/blob/d6314c4c2ef647efe0d12450214fc5b4a4055290/django/contrib/messages/storage/cookie.py#L175

Django Version: 3.2
Python Version: 3.8.2

Traceback (most recent call last):
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/utils/deprecation.py", line 119, in __call__
    response = self.process_response(request, response)
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/contrib/messages/middleware.py", line 23, in process_response
    unstored_messages = request._messages.update(response)
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/contrib/messages/storage/base.py", line 127, in update
    messages = self._loaded_messages + self._queued_messages
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/contrib/messages/storage/base.py", line 79, in _loaded_messages
    messages, all_retrieved = self._get()
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/contrib/messages/storage/fallback.py", line 25, in _get
    messages, all_retrieved = storage._get()
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/contrib/messages/storage/cookie.py", line 86, in _get
    messages = self._decode(data)
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/contrib/messages/storage/cookie.py", line 175, in _decode
    return self.signer.unsign_object(data, serializer=MessageSerializer)
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/core/signing.py", line 195, in unsign_object
    data = b64_decode(base64d)
  File "/home/obs/virtualenv/lib/python3.8/site-packages/django/core/signing.py", line 68, in b64_decode
    return base64.urlsafe_b64decode(s + pad)
  File "/usr/lib/python3.8/base64.py", line 133, in urlsafe_b64decode
    return b64decode(s)
  File "/usr/lib/python3.8/base64.py", line 87, in b64decode
    return binascii.a2b_base64(s)

Exception Type: Error at /user/login/
Exception Value: Invalid base64-encoded string: number of data characters (369) cannot be 1 more than a multiple of 4

(redacted) contents of the 'messages' cookie:

'[["__json_message",0,25,"Successfully signed in as '
 'admin@example.org."],["__json_message",0,25,"Successfully '
 'signed in as jieter."],["__json_message",0,25,"Ingelogd als '
 'admin@example.org."],["__json_message",0,25,"Ingelogd '
 'als '
 'admin@example.org."],["__json_message",0,20,"Bevestigingsmail '
 'verzonden naar test@example.nl."],["__json_message",0,25,"Ingelogd '
 'als '
 'test@example.nl."]]:1lTkj1:j_3PlpYSKiqPTMAB6_p2Q00eE8j6k7n0Sg_-_IpXG7Y')