Error: "Invalid response from server"
The error message "Invalid response from server" can be sometimes puzzling, especially when the actual image exists and looks good when viewed in a browser.
If you receive this error, it might be that there is some output from another plugin (or some notice/warning) that corrupts the JSON answers of the AJAX calls of our plugin. To see this output, you can open the Developer Tools (usually F12, or Cmd-Opt-' on Mac) and check the network calls like in the image below. What you see here is the correct AJAX call:
For other possible reasons, we'd recommend you to take a look at this article: None of my images were optimized, why is that?