Getting a 'remote server returned and error. (400) Bad Request' when setting up an account in Publication Manager

Tridion Docs 14 SP2, Publication Manager 14.0.2

User is setting up a new account in Publication Manager (or Oxygen 21.1) and gets a Bad Request error.

Content Manager access is working.

In the Create an account dialog page the Content Manager web service URL has been verified.

Any idea what could be causing this error?