C# Dropbox.API File Upload Errors

Okay, I'm making headway recoding my project to account for the new mechanism. I'm hitting a stumbling block trying to perform the second half of the initial OAuth2 process. Taking the code that's supplied in the URI and getting the refresh token. Below is a code snippet. I'm using a WebView2 form to run through the process, since it can run as user-level and doesn't require local admin rights.

The choke point is when my routine hits the ProcessCodeFlowAsync() step. It consistently comes back with an invalid_grant exception. I've run across other posts on here where there were multiple calls going on, and the duplication was leading to this. But I'm only initially hitting this step once, when I first launch my app.

Any suggestions on where to go from here? I do appreciate all of your timely assistance. It's been very helpful!

private async void Start(string appKey, string appPwd)
{
this.ApiKey = appKey;
this.ApiPwd = appPwd;
this.oauth2State = Guid.NewGuid().ToString("N");
var authorizeUri = DropboxOAuth2Helper.GetAuthorizeUri(OAuthResponseType.Code, appKey, new Uri(RedirectUri), state: oauth2State, tokenAccessType: TokenAccessType.Offline);
await this.Browser.EnsureCoreWebView2Async(null);
this.Browser.CoreWebView2.Navigate(authorizeUri.ToString());
this.Visibility = Visibility.Visible;
this.Focus();
}

private void BrowserNavigating(object sender, CoreWebView2NavigationStartingEventArgs e)
{
if (!e.Uri.ToString().StartsWith(RedirectUri, StringComparison.OrdinalIgnoreCase))
{
// we need to ignore all navigation that isn't to the redirect uri.
return;
}

try
{
var uri = new Uri(e.Uri, UriKind.Absolute);
var queryParm = uri.Query;
var code = queryParm.Split('=')[1];
var tokenResult = Task.Run(() => DropboxOAuth2Helper.ProcessCodeFlowAsync(responseUri: uri, appKey: this.ApiKey, appSecret: this.ApiPwd, state: this.oauth2State)).Result;
this.AccessToken = tokenResult.AccessToken;
this.RefreshToken = tokenResult.RefreshToken;
this.UserId = tokenResult.Uid;
this.Result = true;
}
catch (ArgumentException)
{
// There was an error in the code being processed.
}
finally
{
e.Cancel = true;
this.Close();
}
}