The Sign in option is disabled when attempting to connect to a portal in ArcGIS Pro

Description

The 'Sign in' option is disabled when attempting to connect to a portal in ArcGIS Pro on the Project tab.

Cause

The ArcGIS Pro registry key for authenticating portal connections is corrupted, and does not reflect the correct authentication registry key value. This occurs when there are multiple portal connections in ArcGIS Pro.

Solution or Workaround

Modify the ArcGIS Pro authentication registry key to allow authentication of the portal connection.

Warning: 
Editing the Windows registry requires administrator privileges on the machine, and must only be done as a last resort. Backup the registry before performing the steps below. Consult a qualified computer systems professional, if necessary.
Esri is not liable for incorrect modifications following these instructions.

1. Navigate to the Windows Start menu. In the search bar, type regedit and press Enter. If prompted, click Yes in the User Account Control window.
2. In the Registry Editor window, navigate to the following folder path:

Computer\HKEY_CURRENT_USER\SOFTWARE\ESRI\ArcGIS Online For Pro\SignIn\<webadaptorhost.domain.com>

3. Double-click Authentication to open the Edit DWORD (32-bit) Value registry key window.
4. Set the authentication value from 0 to 1 in the Value data field. Click OK.

5. On the ArcGIS Pro Portal tab, click Refresh to refresh the portal list.
6. Right-click the portal, and click Test Availability to test the connection of the portal.
7. Right-click the portal, and click Sign in to connect to the portal.