Hi,
Your website requires a user account to access it.
So I can't check the situation and thus I'm not sure what is the problem as I need more information to confirm my speculations
However, I think it's probably an issue with the SEF and maybe canonical URLs.
Check if you have the "Force canonical URLs on listings" setting activated in the HikaShop configuration and if you do, try to turn it off and then clear your website cache if any.
If the URL to the products details page work after that then it would confirm that it comes from the canonical URLs of the products which are incorrect.
If not, we would need an access to your frontend to check the situation. If you want to provide that privately, you can go through our contact form:
www.hikashop.com/support/contact-us.html