IOSR - 500 error while processing payments

[ttpears]

I'm getting ready to go live so I decided to use a paypal sandbox a test the paypal pro processing, after filling in all the user data & credit card information, clicking Submit gives me a 500 error with the following address: components/com_reservations/pm/pppro/process.php

I have a feeling I'm missing a simple step, any ideas?

[CREATIVE Options]

proper CHMOD for the gateway files

[datahell]

I guess you have suPHP.
As Sirigos told you chmod gateway files to 644 and parent folders to 755.