We very recently discovered a bug in classic ASP (asp.dll) you get installing the Service Pack 2 for Windows 2003; removing the SP2 makes it work again as expected, so this is a regression bug introduced by the service pack itself. Basically with SP2 installed, the Session_OnEnd() method in classic ASP is not fired anymore, thus breaking all applications which rely on that event to run cleanup code.
We already have some reports from customers, either directly to CSS or in Newsgroups. I know my colleagues are already well on this, a private fix has been produced and is being tested internally (as far as I know it works) and we are writing a KB article to document this.
The process is still ongoing so I don't have (and can't give you) further details on this, but I guess it's a matter of days before this will be publicly available; of course I'll update this post when we'll have the KB available.
The KB article is still a draft and not public yet (should be released soon, anyway) but the fix is ready; you can get it calling CSS and ask for the hotfix #934903.
The KB article has been published: http://support.microsoft.com/default.aspx?scid=kb;EN-US;934903