Originally Posted by kovidgoyal
Dont use a section of a larger html file as a cover, and if you must do that, then don't mark it as a cover in the <guide>. If you mark it as a cover, it will be removed by the conversion when it is processing covers. Remove the cover entry from the guide and the conversion should work.
Thanks, Kovid, I'll give that a try tomorrow.
I have that entry in the guide for a reason; but my senior memory is a bit vague. I think it was to try to get the cover to show in iBooks, or when I was trying to get old mobi files to open at the cover. I wonder if I might need to leave it in the ePub version but comment it out before conversion. It certainly doesn't cause any problems when I convert ePubs to old mobi.
Anyway, I'll try to let you know tomorrow.