On a new website that we are developing with Joomla 3.9 and JCE pro 2.8.17 we have installed JCE MediaBox 2.1.1.
The JCE Mediabox popup does not open, when you click on the thumbnail the image opens in the window, not in a popup.
We reinstalled version 1.2.9 and the popups work.
I also tested the beta version (2.1.2 Beta 3) without success.
We do not really know who is blocking JCE MediaBox 2.1.1?
If necessary, I can provide you a access to the website by private message.