I first restored ePO following the restore procedures of KB66616; however, that did not resolve the issue.
I then went through the list of extensions in the ePO console and compared it with ones on our Dev server. They were a match.
Then I went through the list of extensions in the \ePolicy Orchestrator\Server\conf\Catalina\localhost directory and compared them to the same list. Apparently, the upgrade of ePO to version 4.5.4, all of the core extensions were still pointing to the previous 2.0 extension version instead of the new 2.0.3 extension version, i.e. console.xml, ldap.xml, and response.xml. I stopped the services, then edited the xml files with the correct version and restarted the services. I am now able to access the Permission Sets.