Cannot return object with relation as ObjectResult

I have simple controller like this(1st code) and it works perfectly, but when I add .Include to include a relation with other table, then it instantly stops returning proper ObjectResult (despite being filled with data).

Postman returns:

Could not get any response There was an error connecting to http://localhost:51111/data/test. Why this might have happened: The server couldn't send a response: Ensure that the backend is working properly Self-signed SSL certificates are being blocked: Fix this by turning off 'SSL certificate verification' in Settings > General Proxy configured incorrectly Ensure that proxy is configured correctly in Settings > Proxy Request timeout: Change request timeout in Settings > General

No exception is occuring, nor no field is null.

public IActionResult test()
{
    var user = _context.UsersTable
               .SingleOrDefault(p => p.Id.ToString().Length > 0);

    return new ObjectResult(user);         
}

public IActionResult test()
{
    var user = _context.UsersTable
               .Include(x => x.Items)
               .SingleOrDefault(p => p.Id.ToString().Length > 0);


    return new ObjectResult(user);      // data IS in the user, even relational collection is filled with data, but it just does not return.   
}



public class User
{
    [Key]
    public Guid Id { get;  set; }
    public virtual ICollection<Item> Items { get; set; } = new HashSet<Items>();
    protected User()
    {

    }
    public User(string login, string password)
    {
        (...)
    }
}


public class Item
{
    [Key]
    public Guid Id { get; set; }

    public string ItemName { get; set; }

    // Relation to User
    public Guid UserId { get; set; }
    public virtual User User { get; set; }

    protected Item()
    {    }

    public Item(string name, User user)
    {
        Id = Guid.NewGuid();
        UserId = user.Id;
        ItemName = name;
    }
}

public class AppContext : DbContext
{
    public AppContext(DbContextOptions<AppContext> options) : base(options)
    {
        Database.SetCommandTimeout(20000);
    }

    public DbSet<User> UsersTable { get; set; }
    public DbSet<Item> ItemsTable{ get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {

    }
}

So far I tried:

user.Items = user.Items.ToList();