I worked with an API technician from Dropbox who determined that the OAuth2 Login box that is generated by BestSync that asks the user to enter their Dropbox email and password, was designed to only permit "personal" Dropbox accounts to be used with BestSync. He said the setting should be changed from "personal accounts" to "all kinds" of accounts. He said this the reason that my Dropbox Business account is not permitted to be used by BestSync.
I provided the API technician with screen shots of the OAuth2 Login window and he asked me to send him the link within the properties of the BestSync OAuth2 Login window. His response to what the properties displayed is shown below: (Quoted from his message)
Thanks! That's what I expected. The 'require_role=personal' parameter is a feature that allows the app to specify what kind of accounts it wants to
allow to link to the app. The 'personal' value prevents work accounts, such as your Dropbox Business account.
If the developer of the app wants to let Business accounts such as yours link to the app, they will need to remove that parameter. (The default is to
allow all kinds.) You may want to contact them to request that they make that change.
Please let me know if you would like me to send you screenshots or anything else you may need for this fix.
Thank you for your assistance.