The scenario itself seems similar to this one where we also had a completely empty database file. I’m not sure how exactly that happened, but the likely reason is that a new project was created and then copied (outside of Tropy, maybe in relation to the fact that the file was in a Dropbox folder) while it was still in use. In the end the ‘real’ project file was found and I’m not sure how exactly the empty project file came into existence. Without having looked at your file I would guess that something like this might have happened here too.