Failed to load viewstate on IE 8 only on Post back with web application on ii 6

Hello,
 
on post back we are facing below error only on IE8 and not in IE9, FireFox -
Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request.
 
Refer below details about ASP.NET 3.5 Web Application.
1. We have previously project structure as below-
"TestProject" is Web Project:
page1.aspx
page2.aspx
page3.aspx
 
"Sample2" is folder and it is part of TestProject Web Project :
page4.aspx
 
2. Modified project structure-
"TestProject"(Web Project)
"AllPages" is folder part of TestProject Web Project:
Page1.aspx
Page2.aspx
Page3.aspx
Page4.aspx
 

Please suggest solution.
 
Thanks.