Hi! 
Conversion failed at my system (latest Calibre version (3.6) at Debian Jessie (oldstable) x64 KDE), showing something about "Loading font: Unknown. Failed to load font: Unknown, drawing text as outlines...".
At this point (at 67% of conversion), the program stuck forever.
The only workaround I found, is rebooting at Windows and running Calibre there.
Is it a bug?
TIA! 
G.