Try conversion from EPUB to EPUB in Calibre. Blog "Świat Czytników" (World of E-Readers in Polish) found out that the*problem's*caused by a*combo*of a language*that's*not supported by Send to Kindle and*extra*attributes in the <dc:language> element.
You can read whole entry here (it's in Polish, but translators these days make it easy 🙂
:
https://swiatczytnikow.pl/wysylasz-p...ne-bledu-e016/