Issues querying MCP server

I’ve enabled the MCP server via the agent plugin, and my LLM can see and detect the list of tools, but whenever it tries to interact with it i get the error below.

Error: invalid session token: failed to decode JSON payload into AppError. Body: Client sent an HTTP request to an HTTPS server. : invalid character 'C' looking for beginning of value

This is a self hosted instance running with TLS with LetsEncrypt certs on the normal 443 port.
Server Version: 11.3.0
Database Schema Version: 148
Build Number: 20708891798
Database: postgres

Has anyone else ran into this issue?

Many Thanks,
Matt