There was an error when attempting to upgrade the existing portal. Failed to start the database server

Error Message

On Linux operating systems, after an upgrade of Portal for ArcGIS 10.5.1 to 10.6 with Insights for ArcGIS 2.1 installed, the Portal Upgrade page returns the following error message:

Error:   
There was an error when attempting to upgrade the existing portal. Failed to start the database server.

Cause

The exact cause of this error is currently under investigation. This article will be updated with further information when it becomes available.

Solution or Workaround

The Portal Service must be restarted for the upgrade to continue normally.

1. Run the stopportal.sh script to stop Portal for ArcGIS.
2. Run startportal.sh to start up Portal for ArcGIS.
3. Open a new browser window and browse to the Portal home page, and continue the upgrade prompts normally to complete the upgrade.