Hi,
I had the same errors after I deleted a couple archives using the mechanism below from manage.py shell.
from hyperkitty.models import MailingList ml = MailingList.objects.get(name="yourlist@example.com") ml.delete()
The errors in the mailman3-web logs were such.
INFO Process-1:1 processing [rebuild_thread_cache_votes] 12:10:34 [Q] ERROR Failed [rebuild_thread_cache_votes] - Thread matching query does not exist. 12:10:34 [Q] INFO Process-1:1 processing [rebuild_email_cache_votes] 12:10:34 [Q] ERROR Failed [rebuild_email_cache_votes] - Email matching query does not exist.
Running the daily/monthly/yearly runjobs by hand did not solve the issue. Then I logged into the django admin from the web interface and deleted the queued tasks with the same names of the processes which were failing. I even deleted the failed tasks, but I am guessing it was unnecessary.
I don't see the errors anymore. But I still have the other issues which were mentioned in other posts that have to do with not updating the number of messages and participants and not showing the messages sent from the email client of the subscribers in the hyperkitty archive.