Serval times this error has been discussed by me, so i decided to share with you. To resolve this issue you have to modify few changes in your web sites to fix this issue.

Here’s is the solution
- Open your Web.Config file.
- Add <httpRuntime requestValidationMode="2.0" /> under <system.web>
- Add <pages validateRequest="false">
You can see the changes in Web.Config as shown.

Note: You can also add validateRequest="false" in your page on the top of the page directive for that page only.
If you have an alternative solution you can comments below.