On the server(s) where WorldServer is installed, we have restarted the Idiom Service (or restarted the server). However, after the restart, the WorldServer User Interface (Login page) cannot be loaded,

To solve this issue, follow these steps:

1- Go to tomcat/conf and open the server.xml file in Notepad++
2- Change the port number 8005 to another number, i.e. 8004 and save your change
<Server port="8004" shutdown="SHUTDOWN">
3- Stop and restart the Idiom (WorldServer) Service. If you cannot stop the service, end the process through the Task Manager. The Idiom (WorldServer) Service will be stopped.



4- Now restart the Idiom Service. A couple of minutes after the restart, the UI will be available again.

Note: if you work on a clustered environment, apply these steps to all application servers except the FTS-only server.

The reason for this behaviour is that port 8005 is in use by a Windows 10 system process. However: this port is configured in the server.xml file for the server shutdown as follows:
 

<Server port="8005" shutdown="SHUTDOWN">
  <Listener className="org.apache.catalina.startup.VersionLoggerListener" />
  <!-- Security listener. Documentation at /docs/config/listeners.html
  <Listener className="org.apache.catalina.security.SecurityListener" />
  -->
  <!--APR library loader. Documentation at /docs/apr.html -->
  <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
  <!-- Prevent memory leaks due to use of particular java/javax APIs-->
  <Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
  <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />

This setting might have been introduced by a Windows update which adds port 8005 to a Windows process, followed by a server or Services Restart. Before the Microsoft update, there was no issue with using port 8005.