This error means that there is an error somewhere within .asp script. To determine what the error is exactly you need the following:
1. Disable custom error do***ent for 500 error.
You can do it directly through Plesk Control Panel.
Plesk CP > domains > domain name > Web directories > Error Do***ents tab > Find Error 500 > Click on it > Select type `Default`.
2.
Disable `Show friendly HTTP error messages` in your browser.
For example in Internet Explorer: Tool > Internet Option > Advanced tab > uncheck "Show friendly HTTP error messages" > Click OK.
Then you can see exact error message and error number. To find out what that error means you need to browse to Microsoft web site and try to search by error number:
http://search.microsoft.com
On dropped down links you can find a solution.