In case anyone needs a solution to this issue (at least for me), I solved it extracting the epub's cover with:
Code:
C:\Program Files\Calibre2>ebook-meta.exe --get-cover=PATH&FILENAME
And then running:
Code:
C:\Program Files\Calibre2>ebook-convert.exe c:\file.epub c:\file.mobi --cover=PATH&FILENAME --output-profile=kindle