I'm getting internal server errors on calls to payment_callback.php that are made from paypal, these errors are being logged in the the access log, but I don't see anything in XenForo Server Error Log in the admin related to them. There are however records for them within the xf_user_upgrade_log table that flag them as transaction_type: error and message: Request not validated.
I should mention that this is interfacing with the paypal sandbox also. Any ideas on what might be going on here? Thanks.