How to reproduce: refer to the attached project

Workaround: reset the data source
private void Button1_Click(object sender, EventArgs e)
{
	this.RadGanttView1.DataSource = null;

	//TODO: This line of code loads data into the 'WeddingPlannerDataSet.Tasks' table. You can move, or remove it, as needed.
	this.TasksTableAdapter.Fill(this.WeddingPlannerDataSet.Tasks);
	//TODO: This line of code loads data into the 'WeddingPlannerDataSet.Links' table. You can move, or remove it, as needed.
	this.LinksTableAdapter.Fill(this.WeddingPlannerDataSet.Links);

	this.RadGanttView1.GanttViewElement.DataSource = this.WeddingPlannerDataSet 
}