I found the solution. I set EnableSanitization="False"
and it works fine.
Code:
<asp:HtmlEditorExtender ID="HtmlEditorExtender1" runat="server" TargetControlID="txtText" EnableSanitization="False">
<toolbar>
...
</toolbar>
</asp:HtmlEditorExtender>
<asp:TextBox ID="txtText" runat="server" TextMode=MultiLine />