mat41,
Your server is running ASP.NET framework 1.0.
To fix this issue, you have two options.
1. Open SpellCheck.aspx.
Change
<%@ Page Language="C#" ClassName="PopUpSpell" ValidateRequest="False" %>
to:
<%@ Page Language="C#" ClassName="PopUpSpell" %>
2. Just upgrade your server to asp.net framework 1.1.
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