BadNodeIdUnknown (0x80340000) — Node ID is not known to the server

The server parsed the NodeId fine but has no node with that identifier in its address space. Distinct from BadNodeIdInvalid (0x80330000), which means the identifier is malformed.

Start free OPC UA → canonical

Diagnosis

When you hit this

A hardcoded NodeId copied from another machine. Many servers generate namespace indexes at startup, so ns=3 on one PLC is ns=4 on its twin.
Browsing paths cached across a server restart or firmware update that renumbered the namespace array.
A tag that exists in the engineering tool but was never marked for OPC UA publication.

Resolution

How to fix it

Never hardcode the namespace index
Read the NamespaceArray (ns=0;i=2255) at connect time and resolve your namespace URI to its current index. The URI is stable; the index is not.
Browse rather than guess
Walk from the Objects folder to confirm the node exists and get its exact identifier, including whether it is numeric, string, or GUID.
Check publication in the engineering tool
In TIA Portal or equivalent, a variable must be explicitly exposed to the OPC UA server. Existing in the PLC is not enough.

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.