To Prevent from SQL Injections, ASP.NET by default does not allow HTML tags "<",">"
It is recommended to allow this check to happen on each postback.
Sometimes you need to submit HTML Content to your page through Rich TextBoxes or Rich Text Editors. In that case you can avoid this exception by setting the ValidateRequest tag in the @Page Directive to false.
This will disable the validation of requests for the page you have set the ValidateRequest flag to false.
To disable request of all page you’ll need to set ValidateRequest false in your web.config <system.web> section
For .Net 4.0 or higher frameworks you will need to also add the following line in the <system.web> section to make the above work.
<httpRuntime requestValidationMode = "2.0" />
That’s it. Hope this helps you in getting rid of the above issue.