[SOLVED] Estimate Shipping & Taxes no popup

I've been searching for two days to figure out why 'Estimate Shipping & Taxes' isn't working.
I'm still working on a dev site.
Everything works except the button for 'Estimate Shipping & Taxes' when you view the contents of the cart.
No popup appears, nothing happens.
Details: OpenCart 3.0.4.0 and PHP 8.3.14.
Anyone have any ideas?
I did find this online: https://github.com/opencart/opencart/issues/14393
But there is no solution provided there.

Do your products have "Requires Shipping" set to Yes?

Have you tried the default theme?

Have you checked both your OpenCart and PHP error logs and your web browser's developer console?

Forget this topic

I need to go back in time a bit...
The webshop is in the Netherlands, and here we have 'Provinces' (Zones) that are never used in the webshop.
It's a small country. No shop asks for the province (zone) to be filled in.
Actually, OpenCart should have a setting like: 'Use Zones for whole shop: Yes/No.'
That's why I started removing all the code related to Zones from OC. It works fine everywhere, except for the 'Estimate Shipping & Taxes.'
I had already forgotten that I had ever removed that.
I left it as it was and removed it via the settings. Extensions > Extensions > Order totals > Shipping > Shipping Estimator: Disabled.