If you have made increased security settings in your network, you would also have to activate the following ports in the network firewall for Sdui videoconferencing. 

Note: If there is only one participant in the conference whose network Sdui is blocking the domain call.sdui.de , then all other participants in the conference are also affected. In concrete terms, this means that if only one person is in the school network and the following ports have not been activated, then the connection is also blocked for all other participants.

For the domains: call.sdui.de