He leído esta publicaciónDbContext ha sido eliminado y autofac pero sigo recibiendo el mismo error:

La operación no se puede completar porque el DbContext se ha eliminado.

public class EFRepository : IRepository
{
    private EFDbContext context;

    public EFRepository(EFDbContext ctx)
    {
        context = ctx;
    }

    public TEntity FirstOrDefault<TEntity>(Expression<Func<TEntity, bool>> predicate, params Expression<Func<TEntity, object>>[] includes)
        where TEntity : class, IContextEntity
    {
        IQueryable<TEntity> query = includes.Aggregate<Expression<Func<TEntity, object>>, IQueryable<TEntity>>
                     (context.Set<TEntity>(), (current, expression) => current.Include(expression));            

        return query.FirstOrDefault(predicate);
    }
}

Y en el Global.asax

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterControllers(typeof(MvcApplication).Assembly);

builder.Register<IRepository>(c => new EFRepository(new EFDbContext()));

ILifetimeScope container = builder.Build();
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));

Inyección de controlador:

public class AccountController : Controller
{
    private readonly IRepository repository;
    private readonly IMembershipService membershipService;        

    public AccountController(IRepository repo, IMembershipService mmbrSvc)
    {
        repository = repo;
        membershipService = mmbrSvc;
    }
    [HttpPost]        
    public ActionResult Login(LoginViewModel viewModel)
    {
        if (!ModelState.IsValid)             
            return new HttpStatusCodeResult(HttpStatusCode.BadRequest);            

        string returnUrl = (string)TempData["ReturnUrl"];
        LoginDto accountDto = viewModel.GetLoginStatus(repository, membershipService, returnUrl);
        string accountDtoJson = JsonHelper.Serialize(accountDto);

        return Content(accountDtoJson, "application/json");
    }
}

Luego, en LoginViewModel:

public LoginDto GetLoginStatus(IRepository repo, IMembershipService mmbrSvc, string returnUrl)
    {
        repository = repo;
        membershipService = mmbrSvc;

        User user = repository.FirstOrDefault<User>(x => x.Username == Username, x => x.Membership);
    ............
    ............
    }