Stuck on final step of checkout

[pseconds]

Hi, using 2.0.18a on Joomla 2.5.8, when someone checks out, and finalizes there order, the order gets added to the order database, but the customer sees a blank page and notification emails don't get sent.  This appears to be happening on whatever payment gateway I choose.  Not sure how to troubleshoot this. 

[jenkinhill]

Check your server error log file as it may be a 500 error.