Did a password reset and the /auth/password endpoint returned this extremely unhelpful "error message".
Instance: https://mastodon.club (v1.4.1)

Not really sure how to repro this one, sorry! It happened once, I tried to force a repro but it worked fine the second time around (about 10 minutes later).