When IIS is not configured properly after creating SharePoint application and there are web parts or features that use session you may get the following error: Unexpected error occured: System.Web.HttpException: Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the \\ section in the application configuration.
As a solution you must register session state handler module in IIS.
- Open Internet Information Services manager.
- Move to site where problem occurred.
- Open modules list of this site.
- Add session state handler like show on the following image (click on it):
- Click OK, recycle application pool and refresh the page.
I am not very sure why SharePoint doesn’t enable this module by default as mostly default session state module is enough.