After speaking with my friend @kamsar, he pointed out the CORS origin on the Identity Server Instance needs an update.

The Identity Server doesn't allow logging in from just any url, for security reasons.

* Open the file `C:\inetpub\wwwroot\sc910.identityserver.dev.sc.local\Config\production\Sitecore.IdentityServer.Host.xml`
* Add a new *AllowedCorsOriginsGroupN*
* Recycle the Identity Server Application Pool

Below you can see the new setting added as Group2:
<!-- language: lang-xml -->

    <AllowedCorsOrigins>
      <AllowedCorsOriginsGroup1>http://sc910.dev.sc.local</AllowedCorsOriginsGroup1>
      <AllowedCorsOriginsGroup2>https://sc910.dev.sc.local</AllowedCorsOriginsGroup2>
    </AllowedCorsOrigins>

## Resources

* [Found this link to the 9.1 documentation][1]
* [Configuration patching for Sitecore Host][2]


  [1]: https://doc.sitecore.com/developers/91/sitecore-experience-management/en/configure-a-sitecore-instance-and-sitecore-identity-server.html
  [2]: https://doc.sitecore.com/developers/91/sitecore-experience-management/en/configuration.html