Not sure about your logic, but I would change it this.CurrentUser.GetStringValue("CustomerName", "undefined") to uInfo.GetValue("CustomerName"). For example:
protected void Page_Load(object sender, EventArgs e)
{
uInfo = UserInfoProvider.GetFullUserInfo(this.CurrentUser.UserID);
this.lblActual.Text = ValidationHelper.GetString(uInfo.GetValue("CustomerName"), "undefined");
}
protected void btnOk_Click(object sender, EventArgs e)
{
this.lblError.Visible = false;
this.lblError.Text = string.Empty;
if (!string.IsNullOrEmpty(this.cmbZakaznik.SelectedValue))
{
uInfo.SetValue("CustomerID", this.cmbZakaznik.SelectedValue);
uInfo.SetValue("CustomerName", this.cmbZakaznik.SelectedText);
UserInfoProvider.SetUserInfo(uInfo);
this.lblActual.Text = uInfo.GetValue("CustomerName");
}
else
{
this.lblError.Visible = true;
this.lblError.Text = "Error";
}
}
P.S. Are using e-commerce module by any chance? To work with the customer you need CustomerInfoProvider not UserInfoProvider.