Veyon and Sophos XGS Firewall - help needed on ports and rules

[Tecoft]

Hi @all, I hope you can help me!
I am trying to get Veyon to work in the computerroom at our school. Unfortunately without success so far. I have only installed the master and one client (each with fixed IPs, same Veyon version). Both PCs have the same user with the same password. I have imported the settings read from the master to the client via USB stick. In my unprotected home network, I can open the client's screen from the master and so on. But our school network is protected by a Sophos XGS firewall. Here I can't get any communication between the master and the client. The service is running on both computers, I can ping each other, telnet localhost 11100 returns RFB 003.008 as the only result line

Is there anyone here who has already carried out the installation with a Sophos firewall and can help me with the creation of the rule(s)? Or who can tell me which ports do I have to open for Veyon (inwards and/or outwards)?
I would be very happy about help and tips
Best regards, Susanne