Hi CTietgen,
Please open file \cuteeditor_files\SpellCheck.aspx, ensure that property setting "ValidateRequest="False" has set for the first line below.
<%@ Page Language="C#" ClassName="PopUpSpell" ValidateRequest="False" %>
By the way, what .net framework version application pool you are for your site? 2.0? 4.0?
Regards,
Ken