I ran the page with different browsers (IE, Opera, Firefox), all did not show the page correctly.

Running webpage in private mode worked, thanks

I have upgraded Domoticz to version 2021.1 If I open Zigbee2MQTT menu (from 'Custom' pulldown menu), it shows 'loading' for some time and once ready the checkbox: 'v.1.19.1-dev (zStack12 20190608), Allow new zigbee devices to join' appears but the list with paired sensors is empty. Also the buttons ...