Administration page doesn't show pop-up (overlay) windows When I try to place new block by clicking "Place block" button or when I click "Configure" on the block I see no pop-up window as I see in tutorial videos, like here: https://www.youtube.com/watch?v=D2KvCGGtBuk 3:10 and 4:25. It seems that this problem is persistent in other administration menus (like views, when I try to choose view format). How do I enable them? Why aren't they shown?

I'm on Drupal 8.1.1, with Nginx web server, Ubuntu 14.04 x32, DigitalOcean. There are no errors in status report except not protected configuration files and trusted host settings, which will be solved when site opens to public.
The block layout is in the standard installation of drupal. This popup should work out of the box.

When you press the button "Place block" this ajax is used:

/admin/structure/block/library/mytheme?region=header&amp;_wrapper_format=drupal_modal...

So I would check first if this ajax is fired and second if there are any js errors in the browser debug tool (F12).