As suggested by @Mark last week, I exported my databases mailman3, mailman2web of what was mailman 3.3.0 (or so) and imported these on my new working system.
Apparently, a database migration step is needed to run the new mailman3 with some older databases: because when I want to login to view my mailing lists, I got "An error occurred while processing your request." in the web.
Log files and error mail messages say inter alia:
Internal Server Error: /accounts/login/ OperationalError at /accounts/login/ (1054, "Unknown column 'socialaccount_socialapp.provider_id' in 'field list'"
and
Internal Server Error: /archives/ OperationalError at /archives/ (1054, "Unknown column 'hyperkitty_mailinglist.archive_rendering_mode' in 'field list'")
Please can you assist: How can I add the missing tables, is there an update script (running "python migrate.py" was not fixing this).