there are a large number of potential causes of this (http://support.microsoft.com/default.aspx?scid=kb;EN-US;829743 details the majority of these). We protect against one of the common ones in dotnetnuke by specifing a fixed validationkey to avoid the process recycling issue), but perhaps one of the other issues is causing this. In addition I know of at least one other asp.net 2.0 specific issue that there is no fix for (http://forums.asp.net/p/955145/1173230.aspx), perhaps that's it. Sorry I can't offer any other advice other than trying some of the options suggested by Microsoft to see if your issue is resolvable.

Cathal