Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted. ...

Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted.  This value may indicate an attempt to compromise the security of your application, such as a cross-site scripting attack.  You can disable request validation by setting validateRequest=false in the Page directive or in the  configuration section.  However, it is strongly recommended that your application explicitly check all inputs in this case.