Perhaps someone else can provide more concrete help, but you're running a version of HyperKitty that is at least 2 years old (we stopped supporting Python 2 around second quarter 2018). I strongly recommend upgrading. I understand that this may be extremely inconvenient since you're running an older Ubuntu LTS, but not only have a lot of features been added since then, a lot of bugs have been fixed.
every night at 00:00 i get an email by mailman3:
Traceback (most recent call last): File "/usr/lib/python2.7/dist-packages/django_extensions/management/commands/runjobs.py", line 34, in runjobs job().execute() File "/usr/lib/python2.7/dist-packages/hyperkitty/jobs/sync_mailman.py", line 37, in execute sync_with_mailman()
I don't recognize this issue, but this looks dangerous. I believe that this process keeps HyperKitty's database synced with Mailman's, and if this process is consistently failing, they could become unsynced and HyperKitty's archive could become corrupt or report incorrect information.