It seems silly to ask, because there would already be a question here because it would affect very many people, but maybe I'm just too stupid:
If I click on "Close" or "Flag" on any stack exchange site with Safari 12.1.1, I see some animated dots next to the button for a very short time, then nothing happens. If I try and try again, a red message box tells me I have to wait three seconds to open the dialog again. But it never did open!
I allow pop-ups for this site (if the dialogs are pop-ups at all), have no blockers or extensions active. MacBook pro mid-2012, 10.14.5.