URI misatch

Hello,

I code a web service in .net/C# and I use the oauth2.

I succeed to do the authorization :
https://www.dropbox.com/1/oauth2/authorize
with params : client_id, state, response_type and redirect_uri

My redirect_uri is a call to another part of my web service.
Inside, I call the next step :
https://api.dropbox.com/1/oauth2/token
with params : code, grant_type, client_id, client_secretand redirect_uri

It fails. The error message is "redirect_uri mismatch". I've checked multiple times and it is the same in my request and in my dropbox app.

I use two differents uri for the first (authorize) and second (token) request (both saved in my dropbox app), is it the problem?

If I use the same uri, how can I distinct the first and second callback ?

Thanks to your help.