During the development of a site creating directories, iIdiscover that IIS lost his session on each directories modification.
Fortunatly ASP.net provide a service to store session out of your website : ASP.NET State Service.
First step is to start the service and request it to start autmtically.
Two parameters can be set in the registry : the port to use and if the service can be reach from a remote computer.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aspnet_state\Parameters
By default the port is : 42424
Now the service is configured and running, the last step is to add a new section in your web.config file :
...
Of course, each objects store in session must be serializable