Hi Kara,
I created a new thread for this issue in order to confuse people.
The bug was fixed in
https://bt.x-cart.com/view.php?id=44174 (that ticket is in Russian as it was reported by a Russian person).
Anyways, we are preparing an update of X-Payments connector this week.
While it is not ready please try
sku_missing_patch.diff patch.