FORM LOADING PROBLEM

Hi,
I've 2 buttons on my form:
1st load form1.aspx
2nd load form2.aspx
when I press button1 form1 is loading - and now - when I press button2
(while form1 is still loading) I have a error presented below:

Exception Details: System.Web.HttpException: Validation of viewstate MAC
failed. If this application is hosted by a Web Farm or cluster, ensure that
configuration specifies the same validationKey and validation
algorithm. AutoGenerate cannot be used in a cluster.

What to do?