i just fixed it! It was indeed in the first error in the log, but a different solution…
I added the lines
homeassistant:
external_url: "https://<domainname>:8123"
internal_url: "https://<local_ip_adress>:8123"
to the config and after restart it now worked. The first error also gone
apparently this config change is needed after 2024.4.1 when accesssing by https over the internet
thankx for your help!