Or just temporarily disable all your port forwarding and see if dropbox starts working, then at least you can confirm that getting the forwarding rules sorted out is the way to resolve your issue and confirm the root cause. What I’d suggest is to specifically set up the port forwarding only for the FTP server, but see if your router can handle the other two dynamically and also as a test see if Dropbox does work in that configuration.

I have seen a port forwarding rule set up for streaming in my router to the MCM, but it’s a uPNP one which it set up rather than me doing so, and only seems to be there as-needed. With my set-up (no port forwarding specifically set), I can certainly use the mobile apps and stream (although generally I have that turned off as I don’t often use it), and I’ve never tried the remote dashboard as for me it’s too big a security risk (again i have it turned off). You’ll probably need permanent forwarding for the FTP server, but for the other two the router should be able to handle it (presuming you have a capable router, most these days should be).

I have no port forwarding set, but my router has uPNP so where required it can set such things up as needed. The QNAP Dropbox APP can synchronize your Dropbox account and your Dropbox folder on the NAS in both directions and in real-time.

What is a dropbox mirror