The error you are getting is because you have EventValidation turned on, at page or website level. The error happens when you try to immitate an user's event (most propably using javasctipt). This gives asp.net an impression that the event was not actually triggere by the postback control.
Most probable place for this error to happen is when you are trying to create some controls dynamically (and the page is also using Update panel).
If you think you can not change the structure of the application then you may consider disabling Event validation on specific page (but before that identify the pages).