If you get an error when you try to convert the Mobi file to ePub, that suggests that the problem lies with that particular Mobi file.
I'd suggest reporting the bug at
https://bugs.launchpad.net/calibre and attaching the Mobi file to the bug report. Mark the report as "private" so the book is not publicly visible.
Are you able to read the Mobi book on a Kindle (or Kindle App)?