Product page not displaying

-- HikaShop version -- : 2.6.2

If ypu go to this page
www.ecomad.com.au/mmp_upgrade/eco-store/...t-seals/square-vents

There are four products - each in the same Category.

But if you click on the first 2 products (top 2) - there is an error page.
If I click on the bottom 2 products, it goes to the product page as it should.

What setting do I have wrong to cause this?
Thanks

Thanks - that did it.