Screen dims when adding to checkout

Hello,

I'm in the hopefully final stages of basic setup before putting my shop out for use.

When I click "add to cart" the screen dims. The product is added to the cart, but no further action is possible until I click the browser back button, after which the screen lights up again and i can proceed to the checkout process.

avalanchesports.ch

payments still in sandbox mode

Thanks for any light you can shed on this (hehehe)

SOLVED

The option to check VAT was enabled. I suspect this doesn't work for Swiss location. However, once disabled the screen no longer dims.