Has something on your router changed recently? In any case, try setting the
checkbox Tools->Options->Advanced->DNS->DNSSEC->Treat all domain names as
fully qualified (Werkzeuge->Optionen->Erweitert->DNS->DNS Sicherheit->Alle
Domainnamen als voll qualifiziert behandeln).
Not that I know of. But the problem doesn't only occour in this network.
Also the jitsi client on freebsd box works fine in the same networks
where the osx version refuses to connect.
The suggested option has no effect.
Can you please send your log again with said option active? The logs from the FreeBSD client for comparison would help as well.
What I see from you previous log is that Jitsi tries to perform DNS lookups and fails. The only DNS server it sees is 22.214.171.124, from which we get no answer. Falling back to Google's public DNS (on 126.96.36.199) doesn't work either. I also see that you have a few public IPv6 addresses and I'm somehow suspecting that this has some unforeseen influence.