Video issues in Talk

I’m experimenting with Talk and mostly got it working. But when doing Video calls, I don’t See remote Video with external participants . Audio works, my Video is transmitted and participants can See me.
Does anyone have an idea, what I could check or analyze?

  • nextcloud Docker on Raspi 3
  • coturn installed
  • using public IP/domain
  • Not all participants affected
    • e.g. a Call to a User using the talk App on Android Over LTE works
    • calls insider my network work

I would recommend to use the the search function of the forum to find an answer on your question, because as far as I remember this point has already been discussed in other threads. Missing tcp-port forwardings, a n insufficient bandwidth or an ads blocker could e.g. cause problems.