This has happened to me twice now. A web garden, running sessionState in StateServer mode reverts back to InProc mode after certain patches are applied to .NET Framework.
We are in a formal environment with development, internal test, external test, and production, so I cannot control timing or nature of patches. At best, I can document and convey to groups involved, including helpdesk, netops, test, users, etc.
I have been looking for an answer to this all day on Google, MSDN, etc, and can find nothing other than obsolete references to State Service being shut down (we don't have that problem). I would love something in writing, KB article would be best, to explain and/or resolve the problem. FYI, I cannot modify the machine.config files of the servers, some of which are load balanced in production.
The results of this configuration change when it occurs are "symptom rich" - what a doctor once said to me when asked what would happen if a blood clot on a relative's artery were to break free.