We have version 2.3.1 of Responsive Lightbox, which should be the latest version according to this thread. I then tried using the current version of jQuery from jQuery Migrate. (Use current in jQuery Migrate, save, delete cache.)
However, the Black Screen with X problem was there when I did that.
I deprecated the jQuery version in jQuery Migrate (Use Legacy 1.12.4-wp in jQuery Migrate, save, delete cache). This solved the problem (again).
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