Are you able to access the Dropbox site itself from your browser? Do any other devices have this issue as well on your current network? Is it possible to connect to a different network to see if this is related?
The LAN is offered by a public facility in the research institution. And within the same subnetwork, my colleagues are able to connect to Dropbox on both web-based and desktop client. I used the same port and connect to my MacBook, it also works fine.
I have a more specific question. How can I decode log files from Dropbox client?