Not sure which viewstate error you want to know about, but if you are
talking about a webfarm it is often that the viewstate isn't valid when
your pages postback to a different server than the one that generated
the viewstate. The cause here is that each server gets a different key
for its MachineKey entry and these keys are used to verify the viewstate
has not been tampered with (unless you disable this check by using the
ViewStateMac=false. You can either disable the viewstatemac or you can
go into the machine.config and make sure the entry for both machines'
machinekeys are the same (see the link below).