[stumped]

if dropbox was reinstalled and accepted Moon as an approved app, then I very much doubt it is a dropbox fault.

you could go into dropbox via PC , remove moon's permission to use, then connect again from moon+ and go through the approval process again, but I doubt that will change things.

only the app author is likely to solve this, so send him the dropbox backup file and the app version used for backup & for restores, tell us what he says. I am pretty sure there is still a bug report function built into the app, somewhere.
looking at my own dropbox - I have 4 backups, each is a very small single file - under 200k. I have not tried very hard to "read" the contents. they are gibberish in notepad & I don't have notepad++ currently installed. Each file name has a date and a device identifier e.g. 2016-12-24 (SM-T800).mrpro
FYI I keep a copy of my preferred moon+ background image in the same dropbox folder location - as that took ages to find & I'd want it for any new device