The problem seems to be it's trying to load the template file from admin, while I think it should be loaded from catalog.
Why I don't know, and don't have time for looking at it.

The reason for confirmation missing might be something else though, can be as simple as the order hasn't reached the correct status to release the voucher.

Thanks Rune

Yes... should have said this is 1.5.5.

There are not many modifications and extensions. Some may be left over from earlier versions I fear. I disabled a Gift Voucher modification but that is for Order Totals, deleted the Modifications cache... but it didn't solve it.

How can I work out which extension/modification overrides /system/engine/loader.php ? Thanks.

This problem originally was found because the first customer ever to purchase a Gift Voucher didn't get a confirmation and her recipient never got an email. So I wonder if we have a bigger problem.

There is no /admin/view/template/default/template/mail/voucher.tpl in Arastta 1.5.5: https://github.com/arastta/arastta/tree/1.5.5/admin/view/template

Also there is some extension/modification overriding your /system/engine/loader.php, since it's loaded from /system/modification/system/engine/loader.php - so check your modifications/extensions ...

And maybe also clean the cache if you haven't done so after updating etc.