Have set up the PayPal gateway through the Sandbox OK. When I checkout, the "payment" process within PayPal completes just fine. But when I click the "Return to order confirmation" button on the last PayPal screen, I get page catalog/checkout_process.php consisting only of the error message:

Fatal error: Call to a member function on a non-object in /home/www/jumpingfrog/osc/catalog/includes/modules/payment/paypal_ipn.php on line 521

I found a post from 2003 suggesting this error was due to PHP being in safe mode. I checked "Server Info" and my install is NOT in safe mode. The other suggestion was that this was due to session data being stored in files rather than database. I AM storing sessions as files, but before I go to the trouble of changing this, I wanted to consult the experts as to whether this might actually be the problem.

Thanks for reading.