In a course, both weekly and topic ajax move button stops working after a certain point.

I checked and i get this error:

Error: uncaught exception: [Exception... "Component returned failure code: 0x80004003 (NS_ERROR_INVALID_POINTER) [nsIDOMHTMLElement.appendChild]" nsresult: "0x80004003 (NS_ERROR_INVALID_POINTER)" location: "JS frame :: http://<DOMAIN URL>/lib/javascript.php?file=%2Flib%2Fajax%2Fsection_classes.js&rev=179 :: <TOP_LEVEL> :: line 46" data: no]