I just upgraded to Catalina and have the same problem: automatic synching no longer happens, and there is no error indicator. My setup could not be simpler. Just one Dropbox folder (not a symlink) in my home directory. No symlinks inside the DB folder. Real files, and not thousands of them either. Before Catalina, DB operated exactly as advertised. Since Catalina, when I add a new file on the Mac, it is no longer synched to DB and my other devices can't see it, neither does the DropBox App if I run it on the same Mac. I don't even get little blue file transfer indicators. There is no sync happening at all.
If I quit DB and start it up again, the sync then happens. So it's not that DB can't find these files and synch them, it's just not doing it automatically. It only synchs on startup. This is a PITA. It means at the end of a day, or whenever I put some material on DB to be shared, I have to shut'er down and restart to make the files actually shared.
UPDATE: maybe they are right about sym links. I just remembered that my home directory IS a symlink, or at least the path to /Users/Moi involves a symlink. Never think about it because it's a couple of levels up from any folder I routinely interact with! So I moved my DB folder out to a data volume with no links involved, just a mount point and folder hierarchy (using Preferences to relocate it) and that seems to have improved matters. When I copy a new file to DB, it now seems to synch actively and immediately.
Note to others: check not only that your DB folder is not a symlink and doesn't contain symlinks, but that the entire full pathname of your DB folder never involves a symlink. That might be the key. Kind of weird/silly if you ask me, to release an app that gags on symlinks... but if that's what it takes to restore proper function, I don't really care where the actual folder resides.
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