This is often due to an issue with your web farm servers: Setting up web farms. Can you check the Web farm application in the administration interface for this instance to make sure all web farm servers are listed and that they display a healthy status: Monitoring web farm servers? You should have servers for at least the admin site and live site but you may have others depending on your setup. If they do not all display a healthy status or some are missing and you are using the automatic web farm mode, you can try restarting the web farm servers in the System application -> Restart all web farm servers button in the General tab. Then, make a request to both the administration interface as well as the live site and they should hopefully be added and transition to a healthy status within a few minutes.
If they do not and you are still having an issue there, I would recommend checking the Event log to see if there are any errors or more information in there on what is going on.