Failed to acquire token silently - Microsoft Graph API to obtain a user’s outlook groups

I am trying to access the Microsoft Graph API to obtain a user’s outlook groups.

Here is the code to retrieve the access token:

    public static async Task<string> GetGraphAccessTokenAsync()
    {
        string AzureAdGraphResourceURL = "https://graph.microsoft.com/";
        string signedInUserUniqueName = ClaimsPrincipal.Current.FindFirst(ClaimTypes.NameIdentifier).Value;
        var userObjectId = ClaimsPrincipal.Current.FindFirst("http://schemas.microsoft.com/identity/claims/objectidentifier").Value;
        var clientCredential = new ClientCredential(SettingsHelper.ClientId, SettingsHelper.AppKey);
        var userIdentifier = new UserIdentifier(userObjectId, UserIdentifierType.UniqueId);

        AuthenticationContext authContext = new AuthenticationContext(
            SettingsHelper.Authority, new ADALTokenCache(signedInUserUniqueName));

        var result = await authContext.AcquireTokenSilentAsync(AzureAdGraphResourceURL, clientCredential, userIdentifier);
        return result.AccessToken;
    }

The method uses a settings helper as follows:

    public class SettingsHelper
{
    private static string _clientId = ConfigurationManager.AppSettings["ida:ClientID"];
    private static string _appKey = ConfigurationManager.AppSettings["ida:Password"];

    private static string _tenantId = ConfigurationManager.AppSettings["ida:TenantID"];
    private static string _authorizationUri = "https://login.windows.net";
    private static string _authority = "https://login.windows.net/{0}/";

    private static string _graphResourceId = "https://graph.windows.net";

    public static string ClientId
    {
        get
        {
            return _clientId;
        }
    }

    public static string AppKey
    {
        get
        {
            return _appKey;
        }
    }

    public static string TenantId
    {
        get
        {
            return _tenantId;
        }
    }

    public static string AuthorizationUri
    {
        get
        {
            return _authorizationUri;
        }
    }

    public static string Authority
    {
        get
        {
            return String.Format(_authority, _tenantId);
        }
    }

    public static string AADGraphResourceId
    {
        get
        {
            return _graphResourceId;
        }
    }    
}

This is the error that I get: Failed to acquire token silently. Call method AcquireToken

Exception Details:

    Microsoft.IdentityModel.Clients.ActiveDirectory.AdalSilentTokenAcquisitionException : Failed to acquire token silently. Call method AcquireToken

The error occurs specifically at this line:

     var result = await authContext.AcquireTokenSilentAsync(AzureAdGraphResourceURL, clientCredential, userIdentifier);

I have checked to ensure that the UserIdentifier matches the value in the cache, but it stills seems to reject the token. Any ideas of where I might be going wrong?