In general, it is not wise to open files directly from servers or dropbox , Google Drive etc.
You are changing it anyhow, so it is of no use when others are opening etc.
Especially if you have Autosave set and files getting bigger.
The best practice, is to download to the desktop and work from there. When done, upload to the server.
That said, probably, the backup file is corrupted, all ready, to.
Try to look for temp files in the ‘working’ files: