Hello @blub
To establish a TLS connection to the HiveMQ Cloud (Serverless), you must ensure that your device/client supports and uses the TLS protocol extension called “SNI” (Server Name Indication). This TLS extension provides the server’s hostname in the “Client Hello” TLS packet.
I have performed some tests with my Serverless broker and I see the same issue you described. While trying with Starter broker where SNI is not required I was able to connect without any issue.
It’s a Postman MQTT client implementation issue. I’m using the latest Postman version 11.7.0.
Kind regards,
Diego from HiveMQ Team