Thank you for your reply, fortunately I could fix it myself.
I’m sorry for being so inaccurate. I think I pressed change on the old domain in the admin panel under the web tab.
Then a popup appeared and “web update” ran. It’s a script of mailinabox. I think it updates the nginx configuration.
However I had to modify the nginx configuration for my specific needs and the script takes the nginx configuration files out of the mailinabox/conf/ directory and overwrites existing changes and therefore breaking my setup.
So I edited the files in mailinabox/conf/ and now I’m good to go again