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Recently we experienced an interesting production problem. This application was running on multiple AWS EC2 instances behind Elastic Load Balancer. The application was running on a GNU/Linux OS, Java 8, Tomcat 8 application server. All of a sudden, one of the application instances became unresponsive. All other application instances were handling the traffic properly. Whenever the HTTP request was sent to this application instance from the browser, we were getting the following response to be printed on the browser.
Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /.
Reason: Error reading from remote server