Validation of Viewstate MAC failed. Using .Net Framework 3.5. Not using form action

I know this question has been asked m开发者_StackOverflowillion times but I think my situation is different here.

I am using .NET Framework 3.5 SP1, there is no form action on the page and it's not happening when I try to submit the form quickly. I am not on a web farm either.

I have a page with two GridViews. One Gridview acts as a master gridview and second as detail gridview. First Gridview has autogenerate select button and clicking select would bring related records in detail gridview at the bottom. I have a refresh Link button at the top. Everything works fine. I click refresh button and it works fine too.

Ok when problem start happening when I leave the page idle for like 10 minutes or so and then I click Refresh button only then I get this "Validation of Viewstate MAC failed" error.

I am scratching my head for the last couple of days but to no help.

Thanks

Viewstate can expire, if your application recycles and you have autogenerated machine keys turned on. Maybe that explains your problem.