Drupal+ubercart session disappears after redirecting to the payment system. How to find the reason?

Question

Drupal

There is a store on Drupal 7 + Ubercart. At some point, information about the payment of orders began to be lost. Those. the payment in the payment system goes through, but the order remains in the in checkout state. In most cases, the payment goes well, but in some cases this problem occurs. Those. recreate the situation does not work. The problem manifests itself when paying through 2 different payment systems (PayPal and another local one), i.e. It is unlikely that the problem is in the payment integration module. modules are different.
It was possible to find out that in a problematic situation, when a user returns from a payment to the site, his session disappears, i.e. $_SESSION array is empty. Please advise how to find and fix the problem.

Answer 1

Never pay through sessions!
Only via POST/GET variables from an external request (rejection from the payment system service or right after the redirect).
All intermediate parameters - pass through payment parameters (order id, for example).