Trying to use OAuth2 for Dropbox in 9 DP 9

[Ben Rubinstein]

I'm attempting to use the OAuth2 and Dropbox libraries in LC 9.0.0 (dp9).

(If I test the Dropbox functions with a temporary access token for my app 
generated from the Dropbox api website, it's fine.)

When I try to use the Oauth2 command to implement this for users, I get a 
sheet dialog with what I take to be a message from Dropbox,
	Invalid redirect_uri. It must exactly match one of
	the redirect URIs you've pre-configured for your app
	(including the path).

The redirect URI I have registered for my app is
	http://127.0.0.1:51051

where 51051 is the port number I've passed in the OAuth2 command.

(As a handy side effect, there's no way to close the sheet dialog...)

This is LC 9.0.0 DP 9, on Mac OS X 10.12.6. I've not yet tried on any other 
platforms.

I'm using
	https://www.dropbox.com/oauth2/authorize
	https://api.dropboxapi.com/oauth2/token
as the pAuthURL and pTokenURL, respectively; with an empty pScopes parameter.

Is there an obvious thing I'm doing wrong?

TIA.

Ben