Exporting InDesign files in epub format.

I'd bet, here, that it's FXL export, which creates page-images in PNG format of all the elements it can't represent with positioned text. There are a number of reasons the error would crop up, but the simplest is that one or more of these background/framing page images is too complex for either the export process or the user's system.

@karljohnson , are you exporting to fixed-page EPUB? And if so, why? It's not a recommended format for anything but a very narrow range of "picture page" books.