>Does manually setting the enableViewStateMac property of the asp.net pages to 'true' have any effect? I read that someone actually fixed it by setting it to 'false' which makes no sense to me. that should be the default. I saw this before and it also may occur when users leave a page open for too long and later generate a post back. The maching key is automatically generated and changes prior to the user causing the postback. If you set a static machine key it may help.

If enableViewStateMac is set to False, which is the default, then changing its setting to True shouldn't be a factor. As it is already set to False, putting a static machine key should also have no effect. There is something else elsewhere that is causing this. I got this twice in four months. So, it is extremely difficult to simulate.