Hey, Gary: so far Dropbox has been the *only* sync service that works for me. I've tried Google Drive, Microsoft's OneDrive, and a couple of others, but those all silently
drop files now and then .
My demands are low, though. I let the night's FITS files accumulate, and then in the morning I drag-and-drop the whole Images/yyyymmdd subdirectory to the Dropbox directory. Works every time for me. But I have no experience with real-time syncing, if that's what you're doing.
Happy new year!
edit: In your place, I would back up the current Dropbox directories and then uninstall and reinstall the Dropbox application. But that's just me...
measuring space rock colors and rotation rates, from Albuquerque NM