CONNACK 1 (0x01) — Connection Refused: unacceptable protocol version

The broker does not support the protocol level the client asked for. The client sent 4 (3.1.1) or 5 (5.0) and the broker will not serve it.

Start free MQTT / Sparkplug B → canonical

Diagnosis

When you hit this

A modern client library defaulting to MQTT 5.0 against an older broker that only speaks 3.1.1.
An embedded or vendor-supplied broker pinned to 3.1 (protocol level 3), which rejects even 3.1.1.
A proxy or load balancer in front of the broker that inspects and rejects the CONNECT packet.

Resolution

How to fix it

Pin the protocol version explicitly
Do not rely on the library default. Set 3.1.1 explicitly when talking to industrial brokers; many are not 5.0-capable.
Check the broker's advertised version
Broker logs state the protocol levels accepted. Sparkplug B is defined over 3.1.1, so that is the safe choice.
Bypass the proxy to test
Connect directly to the broker port. If that works, the middlebox is rewriting or rejecting your CONNECT.

Normalization

What Forge does about it

This is a configuration or addressing mistake on the wire, upstream of anything Forge sees. Where Forge helps is the step after you fix it: once the point returns data, its vendor tag name maps to a canonical field automatically, so you do not also have to hand-maintain a name table for every point you just re-addressed.