Error sending a string to SQL Server

Question

I'm working on a project with C# and SQL Server. I have a table called Area and it has a column called flag, it is a char(1).

I create the object with the following code:

Area objArea = new Area()
{
    CodArea = txtCod.Text.ToString().ToUpper().Trim(),
    NameArea= txtNombre.Text.ToString().ToUpper().Trim(),
    UserCreate= Session.CurrentSession.User.User1,
    DateCreate= DateTime.Now,
    Flag= "A"
};

It worked okay until yesterday but now the program shows this message:

the field flag should be an string or array with length 1.

Does anybody know what is wrong?

I'm using Entity Framework, I catch the exception with the following code:

public void Insert(Area data)
{
        dataBase.Area.Add(data);

        try
        {
            dataBase.SaveChanges();
        }
        catch(DbEntityValidationException ex)
        {
            foreach(var errors in ex.EntityValidationErrors)
            {
                foreach(var error in errors.ValidationErrors)
                {
                    Trace.TraceInformation("Property: {0}, Error: {1}",
                                             error.PropertyName,
                                             error.ErrorMessage);
                }
            }
        }
}

Interesting fact: I have another class called Product and I send the value "A" for flag and it doesnt have any error.

This is the class, is auto-generated

public partial class Area
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Area()
    {
        this.Articuloes = new HashSet<Articulo>();
    }

    public int Id_Area { get; set; }
    public string CodArea { get; set; }
    public string NombreArea { get; set; }
    public string UsuarioCreacion { get; set; }
    public System.DateTime FechaCreacion { get; set; }
    public string UsuarioModificacion { get; set; }
    public Nullable<System.DateTime> FechaModificacion { get; set; }
    public string Estado { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Articulo> Articuloes { get; set; }
}