Can't get collabora Online running

I have NC 20.0.7 with the latest collabora installed.

The strange thing is that it tells me in the settings that the collabora Online Server is reachable (see picture), but I can’t open any file because “server not reachable”.

And yes, I installed the build-in CODE Server, BUT I can’t choose it in the settings, it tells me “server can’t be connected” and after a reload of the page the “use your own server” is marked again. I never chosed to use an own server and I never inserted some URL there. Just installed it and bam it is as you can see it on the picture. What do I have to do?
NC is running Apache2 behind a NGINX Reverse Proxy. But with my former NC instance I didn’t have to change there anything.