Websites are not accessible with HTTP error 503
PleskControlPanel and most IIS application pools could not be started at IIS > Application Pools.
Event Viewer > Application log contains the following error under Event ID 2280
CONFIG_TEXT: The Module DLL C:\Program Files (x86)\IIS\Application Request Routing\requestRouter.dll failed to load. The data is the error.
Microsoft Application Request Routing package corrupted or missing.
Download and install Application Request Routing package from Microsoft website: