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

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