An unknown server-side error occurred while processing the command

When you encounter the error java.util.concurrent.ExecutionException: org.openqa.selenium.WebDriverException: An unknown server-side error occurred while processing the command.

Original error: Could not proxy command to the remote server.

Original error: timeout of 240000ms exceeded.

1. Install Webdriveragent.
2. Kill the running appium proccesses with the following command: killall -9 node.
3. Start the AUT again.