Hey Guys, I am facing the same error but coming from a different place. After trying to signup with any special character (öéÖä) I started to see the error and no user are able to log in even cleaning the local cache.
Access to XMLHttpRequest at 'https://auth-dev. (index):1 [ourDomain]/usernamepassword/challenge" from origin [ourDomain] has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.