"Loading please wait..." message when accessing the server by IP

Hi,
I installed calibre on an Ubuntu 18.04 LTS and started the contentserver.
After accessing the server from another system or by using the IP of the system (on the system the server runs on) the server only displays the "Loading please wait" message.
If I access it using "localhost" instead of the IP it works fine.
I tried it both as a headless server as well as the calibre integrated server.

Can someone please help me to understand why I am getting this message if I use the IP?

Thanks.

Best regards
miracle152004