Login not working in some IE8 versions (It works on all other browsers, Firefox, Chrome, Opera, Safari, IE7 and IE9)

After login attempt, the Internet Explorer cannot display the webpage is displayed.
I load the site again(home page, not refresh) the user is logged in.(the logged out button is displayed).

Also a Page load exception is thrown with InnerException: Validation of viewstate MAC failed.

What causes this? The HTML input buttons from the module configuration/button type drop down. The link button works just fine.

I cannot verify it on the demo site because the link buttons are used there.

Hope this helps somebody!
Manuela