Hi,
The log seems to indicate that there is no user linked to the order being processed.
Because of that, the data sent to PayPal is invalid and PayPal is refusing the payment to be made.
I'm not sure the log is linked to the payment being accepted but the order not being confirmed by HikaShop. It might actually not be for the same order.
If the issue happened for only one order and it works fine for other orders, it's probably that there is a problem with that order in particular. For example, if the user was deleted after the order was created, before it was paid, or if the order was created from the backend, and the user was just never added to the order by the person who created it, or something along these lines.