Editing header & footer stopped working

M1 MacBook Air, Ventura 13.6.4, Numbers 13.2. I have a Numbers document that I duplicate monthly, for over a year. I always open the print preview to edit the footer for the current month. It worked fine last month.

There must've been an update of either the OS or the app, because now I can't type free text in the header or the footer, on existing documents or new blank ones. Also: I can insert page numbers, dates, etc. using the main menubar, but not the Insert command at the top of the print preview window. WTF.