IIS website on a POA-managed Windows Shared Hosting server displays the error 500.19:
HTTP Error 500.19 - Internal Server Error The requested page cannot be accessed because the related configuration data for the page is invalid.
There may be multiple causes for this error, it is not specific to POA and should be investigated in the same manner as on a regular IIS server.
Use the Microsoft article for troubleshooting the error: http://support.microsoft.com/kb/942055/en-us.
One of the most common causes is the error #9 in the article above, the site's web.config is trying to redefine a section that is locked in the global ApplicationHost.config file. Other cases can also happen.
Try to open the site in the Internet Explorer on the web server where it is hosted to see the detailed information about the error and follow the resolution steps described in the Microsoft article linked above.