Hello:
The StatusCode: 0x80040000 [BadResourceUnavailable] indicates that the requested resource is currently unavailable. In the context of the OPC UA protocol, this could mean that the node you are attempting to read is currently unavailable, or the current state of the server does not permit this operation.
If you send a ReadRequest immediately after the server restarts, it’s possible that the server has not fully initialized yet, and therefore cannot respond to your request. In this situation, you might need to wait for a while after the server restarts, and send the ReadRequest after the server has completed its initialization.
As a client, after receiving this error message, you should wait for some time and then re-request the data.