Blynk admin UI not available from device running the server

To increase stability and availability I’m switching to a local Blynk server. Yesterday I successfully installed the local server on a Raspberry Pi 4 and all is working well except that I cannot access the admin UI from the RPI4 but can from another machine on the same network.

So to put it into details:
When I visit RPI4.IP:9443/admin from a pc that is connected to the same LAN as the RPI4, I can view the admin UI without any problems. When I visit localhost:9443/admin or directly from the RPI4 browser I get a “ERR_EMPTY_RESPONSE”.

The has been configured to allow all IP’s ( to connect with the admin UI.

Anyone knows what the problem is and what a potential fix could be?