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 (optionally use a pipe delimiter pointed out by @monkey-dsc)
- Recycle the Identity Server Application Pool
Below you can see the new setting added as Group2:
<AllowedCorsOrigins>
<AllowedCorsOriginsGroup1>http://sc910.dev.sc.local</AllowedCorsOriginsGroup1>
<AllowedCorsOriginsGroup2>https://sc910.dev.sc.local</AllowedCorsOriginsGroup2>
</AllowedCorsOrigins>