iOS Custom Sign-In fails with 'unsupported URL'

There was one report of something similar with iOS. In that case the Okta reactive-native SDK was also being used, but it sounds like a possible culprit could have been having a debugger open.

I am not sure how easy it would be for you to share a reproducible application with us?
It would still be helpful to see if you see the same issue with our sample app https://github.com/okta/samples-ios/tree/master/custom-sign-in

To troubleshoot further it would be helpful to look at your Org logs as well. If you do not want to share the Org URL in the forum could you open a case with the support team so we can collect that information.