Hi, welcome to the forum!

First things first, before proceeding please make sure you have a .jex backup. You can make it from your desktop.

Next, let's check if you're logged on in your Dropbox account from your Android.

Here, could you clarify

Does Dropbox prompt you to create new account for your cloud?
Or
does Joplin fetch no notes from Dropbox and in general behaves as if your cloud has no notes?