Is your dashboard theme is different from the default page theme? If the themes are not same may be there is some problem with you dashboard theme.
I had similar problem with one particular portlet in dashboard. I got an error when i tried to drag and drop one of my portlet. It shows error in one of the java script files(dnd/prototype.js). When i exclude this particular portlet things worked fine. There were other portlets which worked find with drag n drop. i still have not been able to trace what was wrong with this particular portlet.