ASP.NET session state and multiple worker processes

Having multiple worker processes and using InProc does not seem to be compatible.

See this:

If you enable Web-garden mode by setting the webGarden attribute to true in the processModel element of the application's Web.config file, do not use InProc session state mode. If you do, data loss can occur if different requests for the same session are served by different worker processes.

More than one worker process is a "web garden." In-process session state will not work correctly. You'll need to use either a single worker process for your web app, or use a session state server, or SQL Server for session state.