My payment module is Paypal Payments Standard and the shipping module is Frre Shipping.
I just tried again and the logs appear to be empty. I have error reporting turned on on the server and those logs are empty as well.
The message on the opage reads........

Internal Server Error

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

Please contact the server administrator, 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.

If you want I can email you a test log on so that you can actually see what I am trying to relay here.

If you would like, I can send you a log on to the site and let you see