Scratch the earlier theory about KFX, I just delivered my daughter's whole collection (except the 1 book) down to my Kindle for PC, imported all into Calibre, every single one of them was KFX. I was able to convert them all to Mobi and open the books.
So it just leaves that 1 book that won't deliver, but apparently KFX is not the issue.
ASIN: B003XF1XOQ
ETA: Here's something else weird. If I understand your post correctly about dealing with Kindle for PC (
https://www.mobileread.com/forums/sh...d.php?t=283371), given that I'm running Kindle for PC v 1.24, I should have had to do the command line hack in order to disable KFX so that I could download, import, and DeDRM those other KFX files in Calibre. But I didn't have to. I just imported the KFX directly into Calibre, and ran a MOBI conversion on the files, and it worked. Has something changed, or did I misread your post?