you are blaming Calibre for choking on a a file with a defect
( XMLSyntaxError: Char 0x0 out of allowed range, l
ine 21, column 50 (line 21) ) << FWIW that is a NUL, and has no useful purpose in any ebook
It is a fairly well known fact that many render engines tolerate some defects (because they are so stupidly common

), including the viewer used with Calibre or Sigil. Which is the reason we tell folk VALIDATE to EPUB standards and not rely on 'it worked in Calibre'