Can’t proceed on checkout page (payment methods area grayed-out)

I’ve updated all plugins and wp to the latest version.
The problem is that I can’t make a payment on my site as the payment methods are grayed out regardless of my browsers and OSs. I’ve tried Androids, iOs, Win10 and others along with local network and VPN, but the issue remains the same.

When an item is added to cart, the checkout button takes me to checkout page where I shall fill in some forms of my shipping info. No matter whether the forms are completed or not the order and “proceed to payment” area remains gray. When I started to fill in the forms, the page was fully loaded and then, grayed out.

My payment plugin has been set correctly and API certificates are valid. I also activated some other payment methods for testing purpose. Still doesn’t work.