Changed the URL in the IntroscopeEnterpriseManager.properties pointing to the root of the Enterprise Manager REST API to be a hostname not an IP. Certificate was set to match on hostname not IP Address. Adding a certificate to match on server IP as well would resolve this.
Example: #introscope.webview.enterprisemanager.rest.base=https://x.x.x.x:8444/apm/appmap changed to introscope.webview.enterprisemanager.rest.base=https://hostname,subname.companyname.com:8444/apm/appmap
Important clue: Webview log has errors about "java.security.cert.CertificateException: No subject alternative names matching IP address 10.60.168.39 found