Hello,
We have had a long discussion in the ticket and to be frank it seemed to be a browser issue to us as we were able to login fine in Mozilla all the time and also in IE6 & 7 after we had enabled cookies for the site. We do not see it be an issue we the server but we the settings of your site.
I really hope that one of the ASP experts can explain this well.
Regarding the "validation of viewstate MAC failed", it is nothing but a bug in ASP which can be corrected with few changes in the script. You have have a look at the link below, it is one from the ASP .NET forums.
LINK IS HERE