Upload Failed! (-1) error message keeps appearing...

I have run into that issue again with uploading an updated version of my modern city project to Minecraft Realms.

I have tried closing and reopening the game and the launcher a few times over the course of a few days to try and get this done, but every time I do, it stalls for a few seconds, and then it doesn't even load the single-player version of this project at all, greeting me with the (-1) error message...