The login details could not be validated: Unexpected character encountered while parsing value

OS Name/Version: Ubuntu Server 22.04

Product Name/Version: 2.4.1.2

Problem Description:
Whilst trying to add a target to a controller, I get the following error after pressing ‘next’ at the ‘controller details’ screen.

The login details could not be validated: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.

This is a fresh install of AMP on a fresh install of Ubuntu Server 22.04. The controller node likewise. Nothing else configured, no changes made to either instance of amp.