Redcap, I don't know much about dropbox, but could it be that the paper you have uploaded was protected in some way therefore unable to be shared? Someone else' paper rather than your own?
I have more recently run across problems with Google images under the old picassa system where they have tightened up on security. Where I used to be able to post pictures here with no problems, I cannot now see where I can change to "anyone who has the link" It's either share with everyone on the web, which I'd rather not, or a password is required. All urls now marked with https. I had to open a photobucket account to get round this and am now bombarded with advertising rubbish.