Regarding the workaround on chrome developer tools, can you explain its step by step?
For the purposes of posterity ... if you press F12 in most browsers, you can inspect and manipulate the elements of a page. In this case, if you hit F12 and found the DIV tag with the modal-backdrop CSS class, it has a CSS property called z-index, which controls the layer position of the element. The basic problem here is that both the backdrop and the modal window have z-index values of 1040, which leaves it up to your browser to decide which one is actually in front of the other (I would think in this circumstance, the later element on the page should be treated as "in front" but that's not what's happening), so you can manually manipulate the z-index value to make the page function in your browser. And after confirming that that's the problem, obviously, you can make the changes permanent in the CSS files or whatever.
I did try your custom css workaround, but this bug still persist.
The custom CSS I posted in that other thread would fix the issue on the main forum, but I don't think that the admin panel loads that CSS, so I didn't expect it would fix your problem directly (but you could modify the CSS file for the admin panel).