Why won't "Multiexporter" export any artboards? ("Will export 0 of X artboards")

I use the Multiexporter script (blog article explanation, github link) for bulk exports of PNGs or PDFs from many artboards. It usually works fine, but sometimes it refuses to co-operate, with a message like:

Will export 0 of 10 artboards

...even though there are plenty of artboards with content on them. There's no clue as to what the problem is, except a line about "currently visible layers" - but they do all have currently visible layers. Adding prefixes, suffixes and output directories make no difference.