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 1. Warning: Invalid JSON response”.
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.