Hi @george_s ,
Great that you are interested in MQTT and HiveMQ! Welcome to our community.
You indicated you have enabled SNI, but your log shows different. I can see your client receiving error -323 from the server and then asking to check if your client has TLS SNI extension enabled:
As WoflSSL explained, “-313” means that the server has sent back a Fatal Alert to the client. If this happens after the ClientHello message is sent, this most likely means that the client is not broadcasting support for a cipher suite or extension that the server requires."
So it seems your client does not really have TLS SNI enabled. Did you double-check the “Client Hello” packet in the Wireshark already, how it is shown here, can you provide your screenshot?
Regards,
Dasha from HiveMQ support