This is security in .net to prevent malicious code.
Have you set the region specific web settings properly?
If so try this:
string teststring = "efJeMes<ekeÀe³e&keÀejer DeefOekeÀejer";
using (Label testlabel = new Label())
{
testlabel.Text = Server.HtmlEncode(teststring);
Controls.Add(testlabel);
}