I tried to test your service by sending data from OpenHAB. Basically every mqtt client should be able to connect and send data to your service, but it seems to you do not follow official MQTT specificaiton:
According to that
The first UTF-encoded string. The Client Identifier (Client ID) is between 1 and 23 characters long, and uniquely identifies the client to the server. It must be unique across all clients connecting to a single server, and is the key in handling Message IDs messages with QoS levels 1 and 2. If the Client ID contains more than 23 characters, the server responds to the CONNECT message with a CONNACK return code 2: Identifier Rejected.
So other issue is that there is no Identifier Rejected in CONNACK packet.
I took a network trace and you return:
.... .... 0000 0000 = Connection Ack: Connection Accepted (0)
it is confusing for well implemented clients.