Black video windows

We are using and testing Jitsi now for a while and experience the following problem:

When we have a meeting with more than 3 users, the video window om some users gets black for a couple of seconds. Afther that then come back. A couple of seconds / minute later it happens again.

We’ve checked the load on the Jitsi server and that is fine, CPU (8 cores) is running on max 60%. Also is there enough of RAM free. The bandwith is also fine and a lot of free room.

In our firewall we have open up the following ports:

80 - TCP
443 - TCP
4443 - TCP
10000 - UDP
20000 - 20050 UDP
5222 - TCP (not sure if necessary, see: Configuration of a self hosted jitsi jigasi for incoming calls )
5269 - TCP (not sure if necessary, see: Configuration of a self hosted jitsi jigasi for incoming calls )
5280 - TCP (not sure if necessary, see: Configuration of a self hosted jitsi jigasi for incoming calls )
5347 - TCP (not sure if necessary, see: Configuration of a self hosted jitsi jigasi for incoming calls )

Does more people have this experience, and is there a solution? What can causes this problem?

A screenshot of a moment where the problem was there (blurred the working screens + chat):

Thanks for you help.