A couple of things to try:
- Turn on Debugging for your site, to see if you get a more useful error message
- If you can open up your browser debugging tools (e.g. in chrome: menu, tools, developer tools) and then show the 'network tab', before dragging and dropping, then you may be able to access a fuller error message by examining the raw output that results after the drag & drop.
If you find a better error message, then post it here and we can see if we can figure it out.