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.

Here are some of the usual reasons for this and possible solutions:

  • Images that actually have errors or simply don't exist in the server. A possible solution is to use a plugin like Enable Media Replace to re-upload and replace the images that are broken.
  • A firewall that prevents our servers to access your images. Please whitelist our IPs.
  • Restrictive rules in the .htaccess file. A quick way to check whether .htaccess is the reason for this issue is to temporary change the name of the file to -for example- .htaccess_tmp and then try again to see if the plugin can optimize a newly added image.
  • Wrong WordPress settings. Please check if the URL of your site is correctly set in Settings > General.
  • Unsupported image formats (e.g. .svg).
  • You're using a security plugin that has an option that prevents the images from being properly accessed (e.g. "Long URL strings" option in the iThemes Security plugin)
  • Sometimes 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 cal

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