ArcGIS for Portal 10.3.1 - Sign in page Unauthorized

[JimSchindling1]

When attempting to sign in to our newly installed Portal, the option to 'Create An Account' appears and functions correctly. When the account is created  have full access to the site and can list Groups and previously created Users. However, the site does not allow existing users to sign in. The message 'Unauthorized' appears in red.

Below is a screenshot of what is appearing.

This issue is similar to one that was previously posted but there was no resolution given. I appreciate any advice.

[CodyMarsh]

Hello,

Are you able to sign into the portal bypassing the Web Adaptor? The Url would look like this: https://FullyQualifiedDomainName:7443/arcgis/home.

Are all accounts not able to be signed in(i.e. portaladmin) or just the accounts that are newly created?