Why does UserManager.FindById() return null if I do not relog?

I'm trying to find the user associated with the currently logged on user:

public ActionResult Root(string path)
{
    var id = User.Identity.GetUserId(); //This works
    var currentUser = manager.FindById(id); //This returns null

    return View(db.Folders.ToList()
        .Where(folder => folder.User.Id == currentUser.Id)
        .Where(folder => folder.Path == path));
}

This only works if I do not use the indicated part in my seed method. If I do execute this part, manager.FindById() returns null.

    protected override void Seed(ApplicationDbContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException("context", "Context must not be null.");
        }

        const string UserName = "[email protected]";
        const string RoleName = "Admin";

        var userRole = new IdentityRole { Name = RoleName, Id = Guid.NewGuid().ToString() };
        context.Roles.Add(userRole);

        var hasher = new PasswordHasher();

        var user = new ApplicationUser
        {
            UserName = UserName,
            PasswordHash = hasher.HashPassword("123456"),
            Email = "[email protected]",
            EmailConfirmed = true,
            SecurityStamp = Guid.NewGuid().ToString()
        };

        user.Roles.Add(new IdentityUserRole { RoleId = userRole.Id, UserId = user.Id });

        context.Users.Add(user);

        //If I leave this part out, there are no issues.
        new List<Folder>
        { 
            new Folder{Name = "Test", Path = "", User = user},
            new Folder{Name = "Bla", Path = "Test", User = user},
            new Folder{Name = "Lala", Path = "Test/Bla", User = user}
        }.ForEach(f => context.Folders.Add(f));

        context.SaveChanges();

        base.Seed(context);
    }

EDIT: Starting to narrow it down. If I relog my user, everything works just fine. The active user during testing remains logged in from the previous debugging session.