Error message, “The response is not a valid JSON response.”

Hi Constantinos, That is a peculiar error. It's not likely being caused by WordPress itself, but by a third-party plugin you may have installed and activated on your site. One way to troubleshoot this is to disable all plugins on your site, then try to upload a pic again. If you don't see the error message when you upload an image without any third-party plugins activated, you can conclude that one of the plugins is causing this error. Then, re-activate each plugin, one at at time, and retry uploading an image each time until the error re-appears. That will give you a clear idea of which plugin is causing the error, and you'll be able to reach out to the creator of that plugin to report the error message and get it resolved. Hope this helps!