Thank you, Greg, for your answer.
I am doing that (using the Uri builder etc). Since I am still trying to make it work locally (but without the http://127.0.0.1:52475/ port), I am using the uri given to me by the builder, which is: https://localhost:44380/. I have this address authorized in the dropbox app (with the 'authorize' part too). The redirecting file is in the main directory (https://localhost:44380/MyLookBackPage.html), but this doesn't matter at this point for the following reason: The program is passing the var authorizeUri =DropboxOAuth2Helper.GetAuthorizeUri(...) comand, receiving the authorizeUri bellow:
authorizeUri = https://www.dropbox.com/oauth2/authorize?response_type=code&client_id=mykeyommited_here&redirect_uri=https://localhost:44380/authorize&state=2fa0e30ca34d4387bfefaa5bc288a6b9&token_access_type=offline&scope=account_info.read account_info.write files.metadata.read files.metadata.write files.content.read files.content.write
However, when I call the httplistener (https://localhost:44380/) add prefixes (none), getting https://localhost:44380/, and call for the listener start, the program gets out of the function giving the result uid=null.
Note that when I use the http://127.0.0.1:52475/ port, nothing such happens. After the authorization step I get the authorizeUri:
authorizeUri = https://www.dropbox.com/oauth2/authorize?response_type=code&client_id=mykeyommited_here&redirect_uri=http://127.0.0.1:52475/authorize&state=45673cb4324441e0921db1d93802bf32&token_access_type=offline&sc... account_info.write files.metadata.read files.metadata.write files.content.read files.content.write
which is obviously the same as before, buth with the redirect_uri given by http://127.0.0.1:52475/authorize. The httplistener then uses this uri (instead of the http://localhost:44380/ of the previous test) everything is ok and the System.Diagonstics.Process is called, presenting the authorization page and the Oauth flow proceeds normally, issuing a valid uid (and I can download the file from dropbox to my machine).
From this test, it seems to me that it is the use of http://localhost:44380/, which is the Uri I get from example you cited and which I was previously using. The listener is getting this address and issuing an error (looks like).
Sorry for the extent of the message, but I think it would be usefull to give you a detailed account of the situation. Can you help me?