OWIN Authorization Code flow always resturns "invalid_request" (400 Bad Request)

Question

I'm trying to add authorization code flow to my OWIN based application.

My setup is:

Startup.cs

// Enable JWT OAuth Authorization Server
var opt = new OAuthAuthorizationServerOptions()
{
    TokenEndpointPath = new PathString("/oauth/token"),
    AuthorizeEndpointPath = new PathString("/oauth/authorize"),
    AccessTokenExpireTimeSpan = TimeSpan.FromMinutes(5),
    AuthorizationCodeExpireTimeSpan = TimeSpan.FromMinutes(5),
    Provider = new MyOAuthAuthorizationServerProvider(),
    RefreshTokenProvider = new MyRefreshTokenProvider(),
    AuthorizationCodeProvider = new MyAuthorizationCodeProvider(),
#if DEBUG
    AllowInsecureHttp = true
#endif
};

opt.AccessTokenFormat = new MyJwtFormat(opt);
app.UseOAuthAuthorizationServer(opt);

MyAuthorizationCodeProvider.cs

public class MyAuthorizationCodeProvider : IAuthenticationTokenProvider
{
    private readonly ConcurrentDictionary<string, string> _authenticationCodes = new ConcurrentDictionary<string, string>(StringComparer.Ordinal);

    public MyAuthorizationCodeProvider()
    {
    }

    public void Create(AuthenticationTokenCreateContext context)
    {
        context.SetToken(Guid.NewGuid().ToString("n") + Guid.NewGuid().ToString("n"));
        _authenticationCodes[context.Token] = context.SerializeTicket();
    }

    public Task CreateAsync(AuthenticationTokenCreateContext context)
    {
        Create(context);
        return Task.FromResult(0);
    }

    public void Receive(AuthenticationTokenReceiveContext context)
    {
        string value;
        _authenticationCodes.TryGetValue(context.Token, out value);
        context.DeserializeTicket(value);
    }

    public Task ReceiveAsync(AuthenticationTokenReceiveContext context)
    {
       Receive(context);
       return Task.FromResult(0);
    }
}

While the password grant flow works, as well as it does the refresh token flow, the authorization code flow always returns an "invalid_request" when I request an HTTP GET to

http://host/oauth/authorize?response_type=code&client_id=sample333&redirect_uri=https%3A%2F%2Fhost%2F&scope=access&state=samplee666

I can't even debug my MyAuthorizationCodeProvider (no breakpoint hit in Create/Receive methods)

I am surely missing something big :-) but I can't figure out what....