OAuth2 Redirect Url

It sounds like you're not URL encoding the 'redirect_uri' value, and so the URL parameters on your redirect URI are being sent as actual URL parameters to the Dropbox /oauth2/authorize app authorization page itself, which does not expect those parameters.

You'll need to URL encode your 'redirect_uri' value. For example, for the sample you included here, that would be:

https%3A%2F%2Flocalhost%3A44332%2FLogin%3FCloud%3DDROP%26ClientId%3D393903

Also, note that per the documentation, the app authorization page should be accessed at https://www.dropbox.com/oauth2/authorize (not on api.dropbox.com).