With the new version of fontconfig I experience the same thing that was explained here:
viewtopic.php?f=82&t=36326
that was fixed for XnViewMP but not for XnConvert it seems.
Thanks very much for having a look
Fontconfig Problem
Moderators: XnTriq, helmut, xnview
Re: Fontconfig Problem
Thanks it is working! I pushed the package successfully on AUR