[MM3-users] Re-send subscription response and file permission error in queue

A user sent subscription request to the ..-join@ address of a list. Email domain wasn't set up properly, so response got rejected. Now, if the user tries to send another request, no reponse comes back. In the log I find a python stack trace saying subscription already pending.

Sep 08 10:32:44 2020 (19918) Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/mailman/core/runner.py", line 171, in _one_iteration
    self._process_one_file(msg, msgdata)
  File "/usr/lib/python3/dist-packages/mailman/core/runner.py", line 264, in _process_one_file
    keepqueued = self._dispose(mlist, msg, msgdata)
  File "/usr/lib/python3/dist-packages/mailman/runners/command.py", line 196, in _dispose
    mlist, msg, msgdata, parts, results)
  File "/usr/lib/python3/dist-packages/mailman/commands/eml_membership.py", line 104, in process
    ISubscriptionManager(mlist).register(subscriber)
  File "/usr/lib/python3/dist-packages/mailman/app/subscriptions.py", line 520, in register
    list(workflow)
  File "/usr/lib/python3/dist-packages/mailman/app/workflow.py", line 69, in __next__
    return step()
  File "/usr/lib/python3/dist-packages/mailman/app/subscriptions.py", line 227, in _step_sanity_checks
    raise SubscriptionPendingError(self.mlist, self.address.email)
mailman.interfaces.subscriptions.SubscriptionPendingError

Furthermore I am getting a permission denied error in the attempt of shunting the message, which I don't quite understand because the folder has user list group list and 700 mode:

Sep 08 10:25:29 2020 (19918) SHUNTING: 1599553529.9889646+88be5afa0bf3111a83e68cc97ce2000723ad93fd
Sep 08 10:29:49 2020 (19918) Uncaught runner exception: [Errno 13] Permission denied: '/var/lib/mailman3/queue/command/1599553789.306307+6fee74af69263553b5ffd0e73cc095ab651bd855.pck'
Sep 08 10:29:49 2020 (19918) Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/mailman/core/runner.py", line 156, in _one_iteration
    msg, msgdata = self.switchboard.dequeue(filebase)
  File "/usr/lib/python3/dist-packages/mailman/core/switchboard.py", line 150, in dequeue
    with open(filename, 'rb') as fp:
PermissionError: [Errno 13] Permission denied: '/var/lib/mailman3/queue/command/1599553789.306307+6fee74af69263553b5ffd0e73cc095ab651bd855.pck'

How can I remove the pending subscription, or re-send the response email?

And what could be the cause of the permission error?

Using mailman 3.1.1