AuthClient.login returns an error called "Unauthorized request"

This has just begun to happen in the last few days, and my code has not changed. Did I make a mistake or is this new?

This seems to imply a rule that I have to add to my firebase forge security rules to allow access from the domain I'm working on (in this case localhost), but I don't know where to find the documentation.

Can anyone help? How do I get through this error?