Terastation 6400 Dropbox connection failed

[LINGLING]

Hi, I have my Buffalo Terastation 6400 NAS setup for Cloud storage and would like to have it sync with my Dropbox account. After copying and pasting the B-sync authentication code, I received the following error.  I don't use any proxy server/settings, and the NAS is connected to my router.  I tried turning on the Mircosoft OneDrive sync, it works properly.  My computer is windows 10, and I'm able to browse my Buffalo NAS folders.

I followed the same steps described on Buffalo's knowledge base site https://www.buffalotech.com/knowledge-base/configuring-dropbox-sync-on-a-buffalo-terastation-5000#:~....  The connection with Dropbox seems straightforward, but when I tried it on my end, the error occurred on Step 9. 

'Cannot register the authentication code. Verify that network or proxy server setting is correct'.

I also tried different dropbox accounts, my personal one, my friend's personal plus, and a business Trial account. All received the same error after pasting the Dropbox authentication code (Step 9).

Any ideas? Thanks in advance.