Generally, This problem occurs because the
EnableViewStateMac attribute of the
page element or page directive at the top is set to true by default.
If this doesn't help then:
Viewstate are values of page controls, stored in a hidden field,
that is sent back to the server. It is unlikely a client error until your way of passing values from page 2 and Page3 to page1 are not handled properly.
Look at the following links to make sure environment is fine:
http://support.microsoft.com/kb/829743[
^]
http://bytes.com/topic/asp-net/answers/286631-viewstate-invalid-page-might-corrupted-problem[
^]
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