Reset password isn't working

(Bug, Closed -> Fixed, Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Users and Identity
Summary: Unclear what's going on -- the error message is vague -- but it doesn't seem to be working properly.
Originally noticed by Courtney, reproduced by me.
Does not repro on my dev machine, which is weird and worrying, but seems to happen as described in production.