Plug-in Manager handles some of the web requests using Internet Server Application Programming Interface (ISAPI). ISAPI is not compatible with Apache web server versions 2.0.56 to 2.0.59 and versions 2.2.3 to 2.2.4.
If your Apache web server runs any of the incompatible versions, you can replace it with version. 2.2.25, which is the version that OfficeScan and Plug-in Manager are using. This version is also compatible with ISAPI.