I'm using Calibre 0.8.38 with Ubuntu 12.04.
When trying to convert an epub book to mobi, I got the above message after the conversion failed.
i found this bug report:
https://bugs.launchpad.net/ubuntu/+s...s/+bug/1030544
It contained a suggested workaround, but I don't know enough to try it.
Is there any other way to overcome this problem?