3466467 - Introscope Enterprise Manager webview page cannot be opened due to "HTTP ERROR 503 Service Unavailable"

The webview page of Introscope Enterprise Manager "http(s)://<Introscope Host FQDN>:8081/webview" cannot be opened, error message "HTTP ERROR 503 Service Unavailable" displays on the opened page.

In IntroscopeEnterpriseManager.log, the error message below displays:
Failed startup of context o.e.j.w.WebAppContext@NNNN{Introscope WebView,/webview,file:///usr/sap/<SID>/ccms/apmintroscope/product/enterprisemanager/configuration/org.eclipse.osgi/21/0/.cp/WebContent/,UNAVAILABLE}{WebContent}
MultiException[java.awt.AWTError: Can't connect to X11 window server using ':0.0' as the value of the DISPLAY variable., javax.servlet.ServletException: APM Server API Proxy Servlet==com.wily.introscope.webapps.common.APMServerAPIProxy@23860f12{jsp=null,order=99,inst=true,async=false,src=DESCRIPTOR:file:///usr/sap/<SID>/ccms/apmintroscope/product/enterprisemanager/configuration/org.eclipse.osgi/21/0/.cp/WebContent/WEB-INF/web.xml,STARTED}, javax.servlet.ServletException: APM Server Extensions Proxy Servlet==com.wily.introscope.webapps.common.APMServerAPIProxy@1af74f62{jsp=null,order=99,inst=true,async=false,src=DESCRIPTOR:file:///usr/sap/<SID>/ccms/apmintroscope/product/enterprisemanager/configuration/org.eclipse.osgi/21/0/.cp/WebContent/WEB-INF/web.xml,STARTED}]
...
Suppressed: java.awt.AWTError: Can't connect to X11 window server using ':0.0' as the value of the DISPLAY variable.

Introscope Enterprise Manager 10.X

Introscope Enterprise Manager, Introscope EM, Wily, 503, HTTP error, Service Unavailable, DISPLAY variable, Can't connect to X11 window server, APM Server. , KBA , XX-PART-WILY , Introscope by CA Technologies , Problem