The 500.19 error is caused by the IIS 7.0 feature delegation mode. When a feature is delegated to site owners, and the site owners modify the feature, then their changes are persisted in web.config. If the server administration revokes the delegated management on that feature, then the web site owner has the responsibility of cleaning up the feature details from web.config. Otherwise, all sites that had modified that delegated feature will immediately give an "Http Error: 500.19 - Internal Server Error" message. In order to avoid this issue, we recommend that hosters do not revoke delegated features once they are published to end customers.