I realized after posting my request for help is that I was attempting an online Dropbox update while Dropbox was downloading about 70 files that I had put into Dropbox on other computers. I *suspect* this caused the error 2; if so, the Dropbox software should have prevented the update while it was loading files. All that I did was wait until the file update finished, and then did an offline Dropbox update from the web site. That worked. I do appreciate the thoughful answer I received. Had the offline update failed, I would have followed the steps outlined in the answer.
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