This error is due to application pool of website. You might map wrong application pool to website. Please follow the below steps to check.
- Open inetmgr.exe(IIS).
- Click advance settings of website by clicking advance settings link in right side panel.
- Check application pol of website.
- Now click application pool.
- Double click website's application pool(or right click and properties of app pool).
- Application pool should be same as website name.
