The error is from the elFinder software itself. It means that the connection from the browser JavaScript code did not get a response back from the server side code. More than that I do not know.
When you do the drag, does you filemanager indicate that it is doing a "move" or "copy"? When I try this Windows decides that it is doing a "move". elFinder does get the "folder", but it still refers back to the desktop filesystem, so is useless.
You should try a different browser to be sure it is not a browser issue. Can you upload files?
Other than that visit the elFinder forums and see what you can learn there.