DNS failure on some canonical urls

-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.5.30

A lot of my products, when clicked on in the front end to open the detail page, result in a DNS failure error.
When I empty the field canonical url, they are fine again.

Other products don't have the same problem at once; when I first click, they open the detailpage. But when I try a second time. Same dns failure.

I notice that there is a "/" missing in the canonical url (the first one after the domain .nl)
When I type the missing "/" in the canonical (error) url, the page opens fine.

what is causing this problem and how can i fix it without having to check every product in the frontend?

regards

Suzanne