you could try putting enableViewStateMac="false" on the <pages element in Web.config but it would be best not to do that if possible and you really should not need to do that. Fully patched servers that I'm working with don't exhibit this problem.
viewstate errors can also happen if the user clicks the button to postback before the page is fully loaded in the web browsers. possibly something in the skin could be funky or causing the page load to be slow, so i would try other skins to eliminate that possibility.
on a web farm it is important that all nodes have the same machinekey otherwise viewstate errors can result, but you seem to be working with one server so that should not be the problem, that problem would also not be browser specific.
really the browser specific problems should all be solved by updated browser definition files that should have come from windows update
if there is a proxy server between your web browser and the server, it is also possible that is causing the problem