To reproduce:

public Form1()
{
    InitializeComponent();
    DataTable dt = new DataTable();
    dt.Columns.Add("Col@", typeof(string));
    dt.Rows.Add("Unit");
    this.radGridView1.DataSource = dt;

    this.radGridView1.EnableFiltering = true;
    this.radGridView1.BestFitColumns();
}

Workaround: modify the Name of each column in order to remove the "@" symbol:

foreach (GridViewColumn c in this.radGridView1.Columns)
            {
                c.Name = c.Name.Replace("@", "");
            }