Why would you contact Plesk? The issue with ASP not working in 32 bit app pools would be a configuration issue within IIS and not to do with the control panel you are running.
You should also be running each website in it's own application pool. Your forum website should not be running in the same app pool as Plesk.
The settings that you need to change are in IIS not Plesk. To start with in the IIS Manager check under the ISAPI and CGI Restrictions that ASP is allowed.
You should also edit ASP settings in IIS to send errors to the browser, and then edit the Error Pages in IIS to 'Detailed Errors'. You will then get a descriptive error message that will point to the exact issue rather than the generic 500 Internal server error that you are getting.