Okay, thank you.
I’ve restarted many times with different configs.
I’ll reinstall from scratch.
I’ve purged the docker related stuff. I installed nc with snap and configured perfectly working TLS.
No idea what was wrong with the docker, but I assume it was network related.
Anyway I can play now…
Just as a follow-up: it is since a while also possible to skip the domain validation: GitHub - nextcloud/all-in-one: Nextcloud AIO stands for Nextcloud All In One and provides easy deployment and maintenance with most features included in this one Nextcloud instance.