[SOLVED] Jitsi-meet: only one participant in meeting works, next joining participant kicks previous out

I have configured Jitsi-meet with nginx reverse proxy as detailed in the Archlinux wiki (-git packages), which is basically the manual install provided by jitsi [0].
Both prosody and nginx were already in use before, so the recommended config had to be adapted (presumably with success).

Now, if I enter a meeting with Firefox, I can see myself and the microphone levelmeter also shows activity.
When another participant enters the room however, one can briefly hear sound on both, then the first one loses connection and jitsi tries to reconnect (german: “Leider ist etwas schief gegangen”).

All logs seem fine except the nginx log reports stuff like:

[…] connect() failed (111: Connection refused) while connecting to upstream, client:, server: call.mydomain.tld, request: "POST /http-bind?room=test HTTP/2.0", upstream: "http://[::1]:5280/http-bind?room=test" […]

Prosody is listening on both IPv{4,6} on 5280.
Where do I start to debug this?

[0] https://github.com/jitsi/jitsi-meet/blob/master/doc/manual-install.md

Solved: using a vanilla chromium solved the problem. The heavily modified Firefox session used before still does not work.