[MM3-users] Strange URL in error message

(Debian 12)

I have the problem that the hyperkitty archive is empty. The file /var/log/mailman3/mailman.log show sometimes the message

urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='www.XXXXX-project.demailman3', port=443): Max retries exceeded with url: /hyperkitty/api/mailman/archive (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7fa48b43a910>: Failed to establish a new connection: [Errno -2] Name or service not known'))

(there is a slash missing between 'project.de' and 'mailman3'). My goal is, of course, to have an running archive. But I think the error message could be a hint about the reason)

Do you have an idea what can I do about this?