Invalid JSON response

Good Morning,
I have a problem with my CMS
Where you get this message “DataTables warning: table id = libraryItems - invalid JSON response. For more information about this error, see http://datatables.net/tn/1”.
Running the browser debugger shows that “Synchronous XMLhttpRequest on the main thread is deprecated because of its detrimental effects,” could help me with this. We are already over 1 month trying to solve.

Thankful!!