LoginController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using CPS.UI.WebApp.Models;
using System.Web.Security;

namespace CPS.UI.WebApp.Controllers
{
    public class LoginController : Controller
    {
        //
        // GET: /Login/

        [HttpGet]
        public ActionResult Index(string returnUrl)
        {
            // The login form/page will only be avaialble to user who are not currently logged in.
            // If the user is currently ogged in, redirect them away from the login page.
            if (User.Identity.IsAuthenticated) return RedirectToAction("Index", "Home");

            LoginFormModel model = new LoginFormModel();

            if (returnUrl != null && returnUrl.Trim().Length > 0) model.ReturnUrl = returnUrl.Trim();

            return View(model);
        }

        //
        // POST: /Login/

        [HttpPost]
        public ActionResult Index(LoginFormModel model)
        {
            if (ModelState.IsValid)
            {
                bool isLoggedIn = model.TryToLogin();
                if (isLoggedIn == false)
                {
                    ModelState.AddModelError("LoginError", "Must enter a valid username/password");
                }
                else
                {
                    string returnUrl = model.ReturnUrl;
                    if (Url.IsLocalUrl(returnUrl) && returnUrl.Length > 1 && returnUrl.StartsWith("/") && !returnUrl.StartsWith("//") && !returnUrl.StartsWith("/\\"))
                        return Redirect(returnUrl);
                    else
                        return RedirectToAction("Index", "Home");
                }
            }

            return View(model);
        }

        //
        // GET: /Logout/

        [HttpGet]
        public ActionResult Logout()
        {
            FormsAuthentication.SignOut();
            return RedirectToAction("Index", "Home");
        }
    }
}

LoginFormModel.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;
using System.Web.Security;
using umbraco.BusinessLogic;
using umbraco.cms.businesslogic.member;

namespace CPS.UI.WebApp.Models
{
    public class LoginFormModel : BaseModels.GenericPageModel
    {
        /// <summary>
        /// The default title to use as the page title
        /// </summary>
        private const string defaultPageTitle = "Login";

        /// <summary>
        /// 
        /// </summary>
        public LoginFormModel()
        {
            this.PageTitle = defaultPageTitle;
            this.MetaTags.Title = this.PageTitle;
        }

        [Required(ErrorMessage = "Username is required")]
        public string Username { get; set; }

        [Required(ErrorMessage = "Password is required")]
        public string Password { get; set; }

        /// <summary>
        /// The URL that the user will be redirected to after they succesfully login
        /// </summary>
        public string ReturnUrl { get; set; }

        public bool TryToLogin()
        {
            string username = this.Username.ToLower().Trim();
            string password = this.Password;
            
            if (Membership.ValidateUser(username, password)) // Error in umbraco.cms.businesslogic.member.Member.Save() is thrown here
            {
                MembershipUser m = Membership.GetUser(username);
                if (m != null)
                {
                    FormsAuthentication.SetAuthCookie(m.UserName, false);

                    return true;
                }
            }

            // Return false by default
            return false;
        }
    }
}