This is a random bug.
When dragging an item from the workinglist to the navigation tree, when you drop it in the navigation tree, you have sometimes a alert windows shown in attached screenshot.
This is because when dropping a zone which contains a link, then you are redirecting to the link in question.
Also when dropping the item, a seam remoting call is done (POST). Randomly, the redirection occurs before the return of the POST which seems to create the alert error.
The solution is to exclude the link from the draggable zone which is actually consistent with drag'n drop from the content items.