Hi,
Not sure what happened but the call stack was / is missing from your first message on my end.
Anyways, thanks, that's exactly what we need.
The callstack indicates that the issue comes from the file /media/com_hikashop/mail/order_status_notification.preload.modified.php
This file is an override of the preload section of the "order status notification" email. It is generated by HikaShop when you apply modifications to the preload of the email via the menu System>Emails. That way, when you update HikaShop, your modifications are not lost. The drawback is that you don't get the modifications we apply to the preload, like a modification to make the preload compatible with Joomla 5 which you must be missing.
So, I would recommend renaming that file via FTP, in order to deactivate that override.
Then, you can reapply your modifications to the preload of the email.