"internal server error" is a generic "something went wrong, but we can't display it to the screen for security reasons".
So, look in the /logs/ folder for myDebug-xxxxxxxxx.log files that are generated when the error occurs.
If it's not recorded there, then you might have to ask your hosting company for access to the Apache and/or PHP error logs.
See: http://www.zen-cart.com/content.php?124
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