custom fields box in screen options missing

You wrote in another post “The custom fields box is a default (i.e. not added by this plugin) section in the editor screen (e.g. post editor). If you can’t see it, click on the “Screen Options” tab in the top right-hand corner. Is it ticked? If not, tick it and it will appear.”

Unfortunately, this option disappeared in our multisite installation a year ago and never appeared again. The at the time setup code still works but we would like to update some of it and this is not possible if the fields box is missing.

^^