Ok. You showed me the error about the pubsub host, does it mean it works with the main host (beta.kontalk.net) but not the pubsub host? That would be even more weird since it’s the same server responding.
P.S. it might be related to the pubsub hostname not present in the certificate… I can fix that in the meantime.
P.P.S. May I have hostname and s2s port of a server using Let’s Encrypt that you successfully connect to?