Good evening,
I'm having the exact same problem with 2 epubs with the same kind of error
"calibre, version 7.1.0
ERRO: KFX conversion failed: <p><b>Cannot convert Um Mundo Sem Fim, Volume I</b><br><br><b>Conversion error:</b> E21018: Enhanced Mobi building failure, while parsing content in the file. Content: <
www.ken-follett.com> in file: text\part0002.html line: 8</p>"
Could you please help me solve the problem? It's working fine with other epubs.