Welcome to the Auth0 Community Forum!
We currently have a SDK for Ionic 3:
https://auth0.com/docs/quickstart/native/ionic3/01-login
And there is some community demand and potential support for an Ionic 4 integration:
I found a support case where a wildcard in Allowed Web Origins (CORS) was causing this error. If you dm me your tenant name. I can take a look.
Let me know.
Thanks,
Dan