If the customer already has an active web.config file on the hosting account you will want to edit this file and search the text for “<system.webServer>” as you see to the right.
You will want to add the line of code “<httpErrorserrorMode=”Detailed” />” as seen to the right under “<system.webServer>” to enable detailed errors for the 500 Internal Server Error Message.
If adding this line of code does not show you the actual error on the customers site then the issue with the script is most likely related to the coding of the web.config file. The customer will need to rebuild their web.config file to properly work with the website.
Enabling Detailed Errors on IIS 8 (Plesk)