There was an error when attempting to upgrade the existing portal. Web server SSL Certificate does not match list of certificates

Error Message

After Portal for ArcGIS 10.6 installation is completed, the portal homepage loads in the browser. Clicking the Continue Portal Upgrade button returns the following error message:

Error:   
There was an error when attempting to upgrade the existing portal. Web server SSL Certificate does not match list of certificates.

Cause

The portal keystore contains certificates imported to Portal for ArcGIS. This issue occurs when the portal uses a signed Certificate Authority (CA) that no longer exists in the certificate keystore (portal.ks).

Solution or Workaround

1. Revert back to version 10.5.1 and restore the portal from the latest backup.
2. Sign in to the Portal Administrator Directory. 
3. Navigate to Home > Security> SSLCertificates> Update.
4. In the Web Server SSL Certificate field, type portal.

Note:
The field might already be pre-populated with 'portal', but proceed with step 3. This updates the certificate alias in the portal's internal database. 

5. Proceed to upgrade to Portal for ArcGIS version 10.6.