Plesk fails to work when switched to IIS:
Internal Server Error 500 is displayed and the following can be observed in the Event Log:
ISAPI '%plesk_dir%\admin\bin\php5isapi.dll' reported itself as unhealthy. No reason was given by the ISAPI.
The issue may occur if ping is disabled for the application pool preventing its proper recycling.
Open IIS Manager and right-click on the PleskControlPanel > Advanced Settings. In the Process Model section, make sure "Ping Enabled" is set to "True"