Unable to authenticate as admin

Are you using a self-signed certificate? If yes - this is probably the problem. You can try editing the ApplicationAuthenticationStateProvider.cs file and change this code:

            catch (HttpRequestException ex)
            {
            }

to this

            catch (HttpRequestException ex)
            {
                throw ex;
            }

This will throw an exception and it should appear in the logs (if enabled) or in the Event Log (in Application).

Thank you very much for your advice.
The error message has brought me to the solution:

IIS Manager >IIS >Server Certificates >Create Self-Signed Certificate

The Visual Studion's localhost certificate gives the error:

System.Security.Authentication.AuthenticationException: The remote certificate is invalid because of errors in the certificate chain: UntrustedRoot