actually every aspx request is by default vaidated for attacks ( i.e. script injecton) and if any markup/script is found in a request asp.net doesn't run it.
when you have a simple text in your editor it wont throw any error but if it contains any html/script type text the .aspx will raise the alarm.
although ValidationRequest attribute will solve your problem but you should check the incoming request for such sript injection.
Here is an articles that talks about all.
http://www.asp.net/learn/whitepapers/request-validation[
^]
hope it will help.
Thanks,
Hemant
Edit:
add ValidateRequest="false" to the page directive i.e. first line of .aspx markup
Update:
<%@ Page Title="" Language="C#" MasterPageFile="~/includes/Header_Footer1.master" AutoEventWireup="true" CodeFile="article_add.aspx.cs" Inherits="articles_add"%>
to:
<%@ Page Title="" Language="C#" MasterPageFile="~/includes/Header_Footer1.master" AutoEventWireup="true" CodeFile="article_add.aspx.cs" Inherits="articles_add"
ValidateRequest="false"%>
Thanks,
Hemant