Error when tunnelling to embedded nginx

Hi. I have set up a tunnel toward a couple of web services on my home network. All works fine, and Unraid server, a Wireguard gui.

But when I add the my router web gui (a proprietary cpe from my internet provider) I get an nginx error. “401 Authorization Required nginx”.

Something is causing nginx on the router to throw an error when accessed through the tunnel. The possibility to troubleshoot the cpe is poor as it is a closed device. Anyone that have experienced anything similar and can point me in a good direction?