I am not sure what exactly causing the problem I am experiencing since upgrading to ZC 1.5.1. I must also say the problem does NOT happen with a clean install just my upgraded version.

Here is the scenario:
Find a customer with gift certificate in their account (or email a gift certificate to a customer)
Once the gift certificate is available, order an item that's less than the gift certificate value
During checkout pay entirely for the whole order with gift certificate so the customer doesn't pay anything but the GC value.
Now go to the order page and try to edit that order by clicking on "edit" this is when I get the error.

In my case my url looks like this
Code:
https://www.clevershoppers.com/MYADMINFOLDER/orders.php?origin=index&page=1&oID=172400&action=edit
but the page returns a 500 internal server error with the following:
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.
Unfortunately, I do not have anything in the Zen Cart log or Apache log to help me troubleshoot what's causing this.

My order.php is exactly the same as the one in my cleaning install that doesn't have the problem.

I will greatly appreciate any help to trap this error or what's causing it. I am sure this probably NOT an issue related to ZC but I need help to get to the bottom of it. Perhaps something is configured wrong in our server (but then it should have my clean install as well). Probably a mod that I am not aware of? Help please!