My firewall detected traffic on port 5222 (aka. Jabber VoIP-port) the sub-application/protocol detected were QUIC wich if not mistaken is a Google Protocol for quick UDP messaging. The traffic from QUIC was flagged as unsafe/unreliable so the firewall blocked it.
Work around for firewall admins: Open up port 5222 in your firewall and it will work.
This won’t help if your not in charge for your firewall like in a company or on university network. Then you need to consult your IT-department at least for now.