Quantity box stated undefined

[Funtasy]

On any product category list, when clicking in the quantity box the existing 1 text immediately changes to "undefined". The - and + buttons work to change quantity and product is added to cart. This only happens on product category list, not on product details page.

When I click on the quantity box to change the text the number changes to undefined. I also can't leave the page because of it.
I have tried the solutions giving from the other thread but nones work. I have no idea on how to fix this so thats why I ask your help.

Ps. It doesn't happen when I use protostar as template but I have no idea were to search for the problem. Neither for the solution.

Joomla: 3.5.1
Virtuemart: Virtuemart 3.0.16
Link for live test: http://www2.funtasy.be/feestartikelen/per-thema/boerderij/boerderij-feestpakket-large.php

[Jumbo!]

This is a problem of your Joomla! template but not of VirtueMart. If you switch to any other template the issue will be vanished. You need to contact your template developer to fix this problem.

[Funtasy]

Alright, Then it won't get fixed as they will say, ask virtuemart.

[Jumbo!]

As I mentioned earlier - you may always switch to any other standard template (Protostar, Beez3 etc) and verify this.

[jenkinhill]

If you want to prove that it is a problem with your template then try this url, which forces the page to use Protostar. There you can alter the number with no problem.
http://www2.funtasy.be/feestartikelen/per-thema/boerderij/boerderij-feestpakket-large.php?tmpl=component&template=protostar

[Studio 42]

Check for template update, each main release need javascript and code changes.(this is the case in other Ecommerce too)

[Funtasy]

I asked them. Cross your fingers as if I can't find the reason I have to redo my whole website. To much work that I already put into it ...