Tahnk you so much. Yes; that solved the issue. So it was related to the proxy. Sorry for assuming otherwise, just because the content was loaded.
For the sake of completeness I am using Apache as ReverseProxy and not nginx. Here are the relevant configuration lines:
RequestHeader set X-Forwarded-Proto "https"
ProxyPass / http://[::1]:2045/ retry=10
ProxyPassReverse / http://[::1]:2045/
Adding the first line solved the issue.