Gmail authentication is failing with MailKit - gmail

Question

I'm writing an app that reads email folders and messages from gmail.
I started out with sample code I found for using MailKit, and it was working fine for a while.
Then it just started complaining about the scope I was passing to GoogleAuthorizationCodeFlow. I was using #"https:\mail.google.com" instead of "https://mail.google.com/". I don't understand why the scope was working earlier and then just started getting rejected.
But after fixing the scope, ImapClient.AuthenticateAsync() is now throwing an "Authentication failed" exception.
I can't find any detail in the exception as to why it's failing. I checked my settings on Google Cloud and everything seems to be in order.
Can anyone help point me to where I should look to debug the issue?
I don't know why it's failing after working for a while, so I haven't really tried anything yet.
Here's the function that's failing. The last line below calling _client.AuthenticateAsync is throwing an "Authentication failed." exception.
private async Task<bool> connectToGmailAsync()
{
var clientSecrets = new ClientSecrets
{
ClientId = Server.ClientId,
ClientSecret = Server.ClientSecret
};
var codeFlow = new GoogleAuthorizationCodeFlow(new GoogleAuthorizationCodeFlow.Initializer
{
DataStore = new FileDataStore("CredentialCacheFolder", false),
ClientSecrets = clientSecrets,
Scopes = new[] { "https://mail.google.com/" }
});
// Note: For a web app, you'll want to use AuthorizationCodeWebApp instead.
var codeReceiver = new LocalServerCodeReceiver();
var authCode = new AuthorizationCodeInstalledApp(codeFlow, codeReceiver);
var credential = await authCode.AuthorizeAsync(string.Format("{0}#{1}", UserId, Server.Name), CancellationToken.None);
if (credential.Token.IsExpired(SystemClock.Default))
await credential.RefreshTokenAsync(CancellationToken.None);
_oauth2 = new SaslMechanismOAuth2(credential.UserId, credential.Token.AccessToken);
_client = new ImapClient();
await _client.ConnectAsync("imap." + Server.Name, Server.PortNumber, SecureSocketOptions.SslOnConnect);
await _client.AuthenticateAsync(_oauth2);
return true;
}