I just experienced this for the first time. Sucessfully exported several photos to my camera roll (as I have been doing for years with the Dropbox app), and suddenly one got the error. Then all subsequent tries on any other photos got the error. Quit the app, turned phone on and off. No dice. Plenty of space on the phone, internet connection fine, settings in Dropbox to allow use of cellular data.
What solved it was updating the app. Perhaps folks who experiece this while on the latest version would have to actually delete and reinstall the app to get rid of this problem.
