Hi,
Per your callstack, the issue comes from the translation key used on line 16 of the "text" version of the "order admin notification" email.
On my end, I've got this line:
echo "\r\n".str_replace('<br/>',"\r\n",JText::sprintf('ACCESS_ORDER_WITH_LINK',$url));
So, as you can see, there is one parameter for the translation which is $url.
However, if I check the nl-NL translation file of HikaShop, I can see this:
ACCESS_ORDER_WITH_LINK="De besteldetails zijn rechtstreeks te bereiken door op de onderstaande link te klikken na het inloggen op de back-end:"
while in English, I see this:
ACCESS_ORDER_WITH_LINK="You can access the order details directly by clicking on the link below after logging in your back end:<br/><a href='%1$s'>%1$s</a>"
So for some reason, it seems one person who provided translations for the nl-NL language removed the part
<br/><a href='%1$s'>%1$s</a>
that's probably why you're getting that error.
I've updated the nl-NL language file. You can directly modify the language file on your end and it should fix the problem.