Some clients get always disconnected

This is the message some of the clients get :
“Unfortunately, something went wrong. We are trying to fix this. Reconnecting in 9 sec…”

This happens just with 2 of the connecting devices, in these cases laptops with Win 10.

The first client can connect, but no video/audio and disconnects with above message after a few seconds.
The second client does connect, with video and audio, but also gets disconnected with above message after 10 or so seconds.

As they both do not know about anything special could have done, no special browser settings, etc.
I do not know what to look for.

With other clients (Windows, Android, iOS, macOS ) it works as expected.