Did you ever figure this out? Having the same issue.

I figured it out!

Checking the console I found:

TypeError: Cannot read property 'AllowManualLinking' of undefined

Which led me to this issue.

I amended ~/App_Start/UmbracoGoogleAuthExtensions.cs

public static void ConfigureBackOfficeGoogleAuth(this IAppBuilder app, string clientId, string clientSecret,
        string caption = "Google", string style = "btn-google", string icon = "fa-google")
    {
        var googleOptions = new GoogleOAuth2AuthenticationOptions
        {
            ClientId = clientId,
            ClientSecret = clientSecret, 
            //In order to allow using different google providers on the front-end vs the back office,
            // these settings are very important to make them distinguished from one another.
            SignInAsAuthenticationType = Constants.Security.BackOfficeExternalAuthenticationType,
            //  By default this is '/signin-google', you will need to change that default value in your
            //  Google developer settings for your web-app in the "REDIRECT URIS" setting
            CallbackPath = new PathString("/umbraco-google-signin")
        };
        googleOptions.ForUmbracoBackOffice(style, icon);
        googleOptions.Caption = caption;

        var autoLinkOptions = new ExternalSignInAutoLinkOptions(true);  // autoLinkExternalAccount = true
        autoLinkOptions.AllowManualLinking = true;

        googleOptions.SetBackOfficeExternalLoginProviderOptions(new BackOfficeExternalLoginProviderOptions
        {
            AutoRedirectLoginToExternalProvider = false,
            DenyLocalLogin = false, // if ANY external provider has this property set, local login will be disabled
            AutoLinkOptions = autoLinkOptions,
        });

        app.UseGoogleAuthentication(googleOptions);
    }

and it all works as exepcted.

Thanks for these details, saved me a lot of time!

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