There’s not just one Notification channel. Technically there’s two, one for each sendout method. Channels don’t magically are set to inactive. What are you doing?
Please also update your first post for exact versions, thank you.
The SMTP channel was actually missing. Thanks for the hint.
I installed zammad on another machine and recreated the channel using the defaults from there via psql. Now it works.
To the reason:
We migrated from OTRS and deleted groups we didn’t want to use in Zammad. I know, do such modifications at your own risk. I just want to be honest here.
I think we deleted the group that was connected to that channel and the channel with it.