Something is disabled, but what?

I am migrating my J2.5 and VM2.5 webshop to J3.4.1 and VM 3.6.4, first locally on my pc ofcourse.
Also I changed the template form bikestore to storefront.

But I am running into some conflicts now. I think all have the same cause...

The cart in top right corner is working well, if I click on it. A kind op pop-up slides down with the cart content. But without clicking on it, it should display how many items are in the cart and the total amount. Instead of that, it keeps displaying the text 'LOADING'.

When I am on the product detail page, the 'add to cart' button doens't work. Get a lot of error lines about "Undefined index: subtotal". Also the tab/button 'reviews' doesn't work. When clicking on it, nothing happens.

When clicking on the item image, a pop-up has to display. Instead of that the (bigger) image is shown on a new page. Just showing the image and nothing else.

Is this some javascript, jquery, ajax thing what is the problem. How can i fix this. The Storefront demo works fine, so it is no local browser setting.

Oh, I had still some Rupostel One Page virtuemart 2.5 cart update extension enabled

Removing it solves most of the problems, except one: the cart in top right corner still only displaying 'Loading'.
Any suggestions?

Hi

Please make sure javascript compression is disabled and tell me the url to Your site.

I don't use any compression. Everything is disabled.

The new website isn't online yet, only local.

Is there another option?

Hi

Sorry. I can't tell much without seeing it. Let me know as soon as You will be able to show it to me.