Invalid viewstate on certain .net pages

We are getting an Invalid viewstate exception in our .net 2 application. This seems to only happen on 2 particular pages. Page A can have a really big viewstate (around 5MB! I know, it's legacy code), while Page B's viewstate is normal (10k). A user will normally navigate to Page B after using Page A. The thing that we find strange is that we have 3 users who regularly use these pages but we only get the errors from 2 of those users. They all use IE8.

The pages do not use AJAX and we are not using a web farm.

Is the err开发者_运维问答or just down to the ridiculously large viewstate of Page A? If so, why does it happen on Page B too? And why is it caused by 2 particular users?

Thanks in advance

Compressing the 5MB viewstate (as a quickfix) should make it faster and possibly more robust. See http://www.codeproject.com/KB/viewstate/ViewStateCompression.aspx

If the problem is related to sending so much data it may help. (Once the data is on the server it should be ok)