Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience
The script on the other page has set the third parameter of the
.open() method of the
XMLHttpRequest to false, which makes a synchronous request instead of an asynchronous one. If you have access to the the script, you can set the value to true as long as it doesn't affect the sequence of the scripts rendering. This might also be set inside of a number of jQuery AJAX functions. Check the async property for a boolean (true / false); it would be set to false for this error.