Error with aspnet sample

I’ve set up the following sample: https://github.com/okta/samples-aspnet/tree/master/okta-hosted-login

Auth flow works, however when it returns to the redirectUrl (/authorization-code/callback) I get the following 401 - Unauthorized with internal server error:
Okta.AspNet.<ExchangeCodeForTokenAsync>d__5.MoveNext() +1268

Anyone have any ideas?