[MM3-users] Re: HyperKitty async tasks runner: systemd qcluster service fails eventually

I think I have a lead: after having removed Restart=always which masked the details about the systemd service failure, I get:

# systemctl status qcluster
● qcluster.service - HyperKitty async tasks runner
   Loaded: loaded (/etc/systemd/system/qcluster.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Thu 2019-12-05 15:04:06 CET; 2s ago
  Process: 1199893 ExecStart=/usr/bin/django-admin qcluster --pythonpath <django_project_path> --settings settings (code=exited, status=1/FAILURE)
 Main PID: 1199893 (code=exited, status=1/FAILURE)

Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:     configure_logging(settings.LOGGING_CONFIG, settings.LOGGING)
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:   File "/usr/lib/python3/dist-packages/django/utils/log.py", line 76, in configure_logging
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:     logging_config_func(logging_settings)
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:   File "/usr/lib/python3.7/logging/config.py", line 800, in dictConfig
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:     dictConfigClass(config).configure()
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:   File "/usr/lib/python3.7/logging/config.py", line 571, in configure
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]:     '%r' % name) from e
Dec 05 15:04:06 samsung4-ubuntu django-admin[1199893]: ValueError: Unable to configure handler 'file'
Dec 05 15:04:06 samsung4-ubuntu systemd[1]: qcluster.service: Main process exited, code=exited, status=1/FAILURE
Dec 05 15:04:06 samsung4-ubuntu systemd[1]: qcluster.service: Failed with result 'exit-code'.

The file handler is configured that way in settings.py

LOGGING = {
    'version': 1,
    'disable_existing_loggers': False,
    'filters': {
        'require_debug_false': {
            '()': 'django.utils.log.RequireDebugFalse'
        }
    },
    'handlers': {
... 
        'file':{
            'level': 'INFO',
            #'class': 'logging.handlers.RotatingFileHandler',
            'class': 'logging.handlers.WatchedFileHandler',
###            'filename': os.path.join(BASE_DIR, 'logs', 'mailmansuite.log'),
            'filename': '/var/log/mailman3/mailmansuite.log',
            'formatter': 'verbose',
        },
...
    'formatters': {
        'verbose': {
            'format': '%(levelname)s %(asctime)s %(process)d %(name)s %(message)s'
        },
...

Anything suspicious?