Reboot the machine after upgrading if you haven’t done so already, this may fix the issue.
The 503 Error means there is an issue with the application pools, typically this means they aren't running and the most common reason is the credentials.
Log onto the host machine as the Clarity service account
Check the Application Pools Open IIS (Internet Information Services) Select Application Pools on the Left
There are three Clarity application pools Right click on each one in turn and stop the service if it is running
Right click again and go to Advance Settings
Select Identity then select