I tried it on my Oasis 2 running 5.10.3. I manually sideloaded it to the documents directory on the kindle over USB and ejected it. It showed in the home screen and opened fine.
How are side loading the book? I saw another user reported that sideloading via calibre was not working for them.
https://www.mobileread.com/forums/sh...d.php?t=315830