You should open you IIS server and select Application Pools node from the Connections tree. Then on the right screen you will see all available application pools that your server expose. ManagedPipeline Mode column shows the mode of each application pool. Afterward go to your website or virtual directory. Right click on the website or virtual directory and choose Manage Web Site/Manage Application >> Advanced Settings. Then check the application pool under you are running your application pool. If necessary change it. Then restart the server.
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