Ok this thing just never seems to work anymore. I've updated to the new File Provider system, hoping to alleviate some of the pain of using Dropbox on MacOS (I use Ableton a lot and supposedly synced files often, although randomly, end up missing or displaying as 'zero bytes'). Now I've updated to the new File Provide system things are either stuck syncing for literally days, or they magically complete syncing, and then I drag a file in from my hard drive and we're back to syncing forever. It has every firewall exception it needs and full access to my hard drive.
I get that Apple have changed the ways files are accessed by apps a while back, but it shouldn't be this hard!