I am facing this problem in an application for sending email by using EasyWebEditor. This error is comming when I click on send email(Submit) button. I found that the problem is coming due to the validation is on on the page tag.
To remove this problem you have to Disable the ValidateRequest in the Page. For this you have to set ValidateRequest=”false” in the Page Directive(tag) like <%@ Page … ValidateRequest=”false” %>. I am sure that it will remove this error.