Wolpertinger Im relatively new do docker and container stuff.
And then already running a complex groupware solution with 18 containers? Thats a steep learning curve…
Could be a permission issue. You need to tell us your setup, on what server, which operating system, firewalld or ufw enabled (you should not), have you upgraded as root user, etc.etc.
As this error comes in the browser, you should check your nginx logs first:
https://docs.mailcow.email/troubleshooting/debug-logs/
So in this case:
docker compose logs nginx-mailcow