refresh token using C# and all

Dropbox doesn't offer video tutorials for the Dropbox API, but I'll pass this along as a feature request. I can't promise if or when that might be implemented though.

Regarding the issue you're currently seeing, the http.Start() line starts a local HTTP listener to receive the result of the authorization flow automatically. The "Access is denied" error doesn't seem to be coming from Dropbox itself, but rather from your system indicating you aren't able to start the listener. Please refer to your platform/operating system's documentation for information on how to configure your permissions to allow your account to start a listener like that. Unfortunately as that's not made by Dropbox, I can't offer support for that itself.

Alternatively, you can omit the redirect URI and not use a listener to receive the result. The redirect URI is actually optional, in which case you would be presented with the authorization code to copy/paste back into the app manually.