NC18: Group video call does not work

I am trying to run a group video call, but it seems I can only run video calls with 2 people. When a third person joins the conversation, it’s video and audio feed is not displayed, only the participants name.

I create a regular call and send a link to 2 more participants (who do not have an account, but join via public link). The first person can join the conversation fine; the third person can join as well, but audio and video feed do not work.

My system:
NC 18.0.1
OS: Debian stable
Webserver: lighttpd 1.4.53
CPU: Intel® Xeon® CPU E5-2690 0 @ 2.90GHz (1 cores)
Memory: 3.9 GB

Any ideas what the problem might be?

Bump. Anyone got any idea here?