DPS/Payment Express contributions approved, 500 error on return to site

I'm having an issue with DPS/Payment Express. Although I'm reporting it for 4.6.9, I first encountered it when upgrading to 4.6.8 from 4.4.x. The issue is affecting two sites on the same server. Both were successfully processing credit cards through DPS and recording the payments until the upgrade to 4.6.8.

The issue occurs when I complete a payment on the Payment Express hosted payment page, and the payment is approved. I then click Next to return to the confirmation page on the originating website, with a URL such as the following:

http://www.example.com/sites/all/modules/civicrm/extern/pxIPN.php?result=000003002225545000d1987a02e4cc53&userid=DPS_test_id

When this URL is requested, a 500 error is generated.

This was generated using a DPS test account - a transaction using a live DPS account generates a much longer code than this but otherwise has the same outcome (500 error). Consequently, the contribution is not marked as incomplete and the receipt for the contribution is not sent to the donor.

In the Apache error log, I get this line at the same time:

PHP Fatal error: Call to undefined method CRM_Core_Payment_PaymentExpressIPN::singleton() in /var/www/sitename/sites/all/modules/civicrm/CRM/Core/Payment/PaymentExpressIPN.php on line 374

Line 374 contains this code:

$ipn = self::singleton($mode, $component, $paymentProcessor);

I'm stuck at this point.... not sure if it's a configuration issue, server issue or code issue. Thanks in advance for any help.