@kraft @BackwardsFeet @annyr I've got confessions & Mass at my parish coming up, so I've got to step away from this - but I can already report that the formerly non-working post is newly accessible now that we're on PHP 7.4.
The footnotes block in that post displays an error ("This block has encountered an error and cannot be previewed."). Deleting and replacing it doesn't help.
I'll try cloning my original post (w/out footnotes) and re-adding footnotes to see if it tosses the error(s) anew.