Hi,
I've spent a few hours trying to understand the problem.
I've narrowed it down to the line:
echo HTMLHelper::_('content.prepare', $html);
in the view file override product / show.
This is custom code which is not in HikaShop by default.
I've changed it to :
echo JHTML::_('content.prepare',$html);
as it corresponds more closely to what is there by default in HikaShop:
<div id="hikashop_product_description_<?php echo $variant_name; ?>" style="display:none;"><?php
echo JHTML::_('content.prepare',preg_replace('#<hr *id="system-readmore" */>#i','',$variant->product_description));
?></div>
and the error doesn't happen.
It doesn't help that you have a mechanism on your website which makes the page redirects to a custom 404 page whenever there is a PHP fatal error happening and I don't know how to deactivate that.
Ideally, you can look in the PHP error log of your server which should contain the fatal error message.
But my change seems to help so I guess the problem can be marked as solved.