Put the same machinekey on all the servers (usually it happens when application is hosted on multiple servers in a cluster or webfarm)
P.S. - the machinekey will go inside System.web section in web.config.
<system.web>
<machineKey validationKey=’3AE041467624C0947FE562E2DBE6122B532FBC4AA9845AE3D82EC6AC473104EA40A27F8E89BADDAEE44B1F7582FC47299B674ED1FD2DDE1368710504EB2C54C1′ decryptionKey=’2FC79A393A5A463F82D38828F43ED7D030C418B448C9E76C’ validation=’SHA1′/>
</system.web>