It wold appear you must use the Login link at the top-right of https://thimble.mozilla.org/. That sends you to a login page with a bunch of extra parameters after the https://id.webmaker.org/login?.

e.g. https://id.webmaker.org/login?client_id=wm_id_wMvw9BSwq2A480...

If you try to login just from https://id.webmaker.org/login you will get the error you saw.