After migration of your website (from ServerA to ServerB) to a new Webserver when you try to access the website you get “503 Error – Service Unavailable” Error on your website and IIS Management Console shows that application pool of the site is crashed or stopped. Even after you start the application pool of the site it is getting crashed when someone try to access website.
Event Viewer shows following error
This error shows that the your site is trying to access IIS rewrite module which is not installed or configured on your New Server. If you check the IIS Roles and Features you will find that both the server(A & B) have same roles and features installed but still IIS of ServerB don’t have Rewrite module. To fix this problem you have to install IIS Rewrite modules separately. IIS URL Rewrite module enables Web administrators to create rules to implement URLs that are easier to remember and easier to find by the search engines. You can see more information about this module on IIS.NET site or you can directly download them using following URL:
After installation of the module don’t forget to restart IIS Web Server.