Sorry, I don’t use Dropbox but I think you can use Auto-detect (or Help button on Proxies tab).
Question: If you use Proxy settings -> Manual -> Apply, the settings are not permanent?
Maybe you can find the proper answers on Dropbox website (docs, how-to, community, …) or Google search.
or here:
BR,
Gabriel
PS
Or try @giacomo’s answer: Otherwise you can create a proxy bypass for hosts running dropbox client.
Edit:
I think it’s a bug from Dropbox:
