We had a problem with Session states dying, apparently after an
upgrade to using the ASP.NET Framework v1.1, but the problem turns out
to be with our virus checker on the server - NetShield from Network
Associates. As soon as we disabled the virus checker, the session
stopped dying and everything worked fine.

We're now looking at excluding the webroot from the attentions of the
virus checker and this should fix things as well as giving us virus
checking for most of the box.

I suspect it's to do with the "shadow copy" facilities of ASP.NET -
the facility which allows you to copy DLLs without stopping the web
server - which is causing the problem, but in any case try stopping
your virus checker and see if that fixes it.

Cheers,

Colin