SymptomsParallels Plesk Panel 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.
CauseThe issue may occur if the ping of the application pool is disabled in IIS, preventing proper application pool recycling and leading to errors.
ResolutionOpen IIS Manager and right-click on the PleskControlPanel application pool -> Go to Advanced Settings.
In the Process Model section, make sure "Ping Enabled" is set to "True":