@evergreen: Have a look at
this post. According to
mattmc, you might not be able to sideload an AZK file if the .opf file of the source epub contains the invisible Unicode characters, U+2028 and/or U+2029 (line separator/paragraph separator.)
If the source epub is not an epub3 file, try opening and resaving it with
Sigil before converting it with Kindle Previewer. (This'll rewrite the .opf file.)
Alternatively, open the .opf file with a
hex editor and search for the hex strings
2028 and
2029 and replace them with
2020 (regular space).