moritz both writes:
"/home/daneben/.local/lib/python3.8/site-packages/django/db/models/query.py", line 435, in get raise self.model.DoesNotExist( hyperkitty.models.email.DoesNotExist: Problem installing fixture '/home/daneben/mailman-suite/datadump.json': Email matching query does not exist.
I assume you're using "python manage.py dumpdata > datadump.json" to dump and "python manage.py loaddata > datadump.json" to load the data? Are you using the right python binary, ie the one Django uses?
I guess it's possible that some data is not in the database, but retrieved from Mailman core's on the fly, in which case you need Mailman core running.
That said, this is entirely handled by Django, I think, and you might be better off asking on Django channels. I'll give you a couple of wild guesses, but unless somebody else knows more about Django than I do, it's probably fastest to ask on Django channels.
I did a quick search for "datadump.json" and brought up these references, but none seem to refer to Mailman: https://stackoverflow.com/questions/4371375/db-error-when-running-python-man... https://www.reddit.com/r/learnpython/comments/8jkre9/deserializationerror_wh... https://www.rockandnull.com/django-dumpdata/
Here's a tutorial on SQLite to MySQL migration in Django: https://newbedev.com/what-s-the-best-way-to-migrate-a-django-db-from-sqlite-...
These are tutorials on SQLite to PostgreSQL migration in Django: https://www.solution-hub.com/Blog_description/37/ https://dev.to/thepylot/how-to-migrate-data-from-sqlite-to-postgresql-in-dja... https://gist.github.com/sirodoht/f598d14e9644e2d3909629a41e3522ad https://simpleit.rocks/python/django/migrate-django-sqlite-to-postgresql/
Perhaps one of those references will help.
Steve