500 error after customers completes paypal transaction

I have a site where I sell downloads. When I tested it, it worked fine. Nothing new added or changes besides updates. First customer emails and says site crashed after payment. Payment did go thru, but did not allow download. I set up a test item for 1.00 and had same thing happen. 500 Error, refreshed an was back at checkout page. Access has been granted, TIA!