Same issue after upgrading to 5.6. When clicking on any site link, the page displays nothing but a black background with a white X in upper right.
However, as a quick fix go to Lightbox | General and change the Default lightbox to prettyPhoto or another preferred light effect that is not SwipeBox.
My guess – this appears to be an Ajax/JQuery issue within WP not necessarily the the author.
So just try changing the Default lightbox until there’s a fix.
worked like a a charm !!! thx !!
