I only encountered the problem when I uploaded as .kepub.epub - it appears that the renderer for simple EPUBs is more forgiving over this issue than the one for KEPUBs so the already broken EPUB displayed o.k. That's why I need to give my library a check before loading any more, not because I thought there was a problem with the extended driver.
I did think the problem was in the original EPUB conversion, possibly because this had been done a couple of years ago with an earlier version of Calibre but never though that it would be cover related.
BobC