Occasionally users at our site are getting "Could not find a valid session key" messages when making payments through PayPal Standard. The payment succeeds but this message unsettles users.
The sequence of requests for a contribution is ...
- POST /civicrm/contribute/transact (submit payment details)
- GET /civicrm/contribute/transact?_qf_Confirm_display=true&qfKey=XXXXX
- POST /civicrm/contribute/transact (confirm payment details)
- POST /civicrm/payment/ipn/NNN (PayPal IPN callback)
- POST /civicrm/contribute/transact?_qf_ThankYou_display=1&qfKey=XXXXX
On the occasions when this fails the 5th step says "Could not find a valid session key". I'm unable to reproduce this on the PayPal Sandbox.
I'm running CiviCRM 5.10.4 (with security patches) on Drupal 7.67.