Magento HTTP Error 500 on Success Page

Getting a HTTP Error 500 (Internal Server Error) on the success page after a successful checkout.

Th开发者_如何学运维is is after an upgrade from 1.4.0.1 to 1.5.1.0.

This might be due to a template issue .. Maybe something change in the XML?

Any ideas would be greatly appreciated.

It's likely you've got PHP locked down to serve a 500 on a fatal error. Your best best is to check the error log for your web server.