In case other people have the same issue, here is how I resolved it.
We did same upgrade in our Development and Staging environments, did not have this issue. But in Production, after the Portal upgrade setup.exe finished, clicked Finish to close the installation wizard. The portal website opened in a browser window without anything displayed. Wait for >10mins, still nothing was displayed. Checked the Task Manager and noticed PostgreSQL was not started. Restarted server did not fix it. From Portal admin REST API, ran Upgrade, this issue got fixed.
Rose