Setting EnableViewStateMac=true is a security measure that allows ASP. NET to ensure that the viewstate for a page has not been tampered with. If on Postback, the ASP. NET framework detects that there has been a change in the value of viewstate that was sent to the browser, it raises an error - Validation of viewstate MAC failed.

Use <%@ Page EnableViewStateMac="true"%> to set it to true (the default value, if this attribute is not specified is also true) in an aspx page.