It's bug in the version of libxml2 on your system. It's been fixed before. and has resurfaced again. Unfortunately, no one can seem to get the maintainer of libxml2 to take any notice.
In the meantime, I've created a bug free version of libxml2 that can be installed alongside Sigil (does not affect the system version of libxml2 at all). There's a sticky thread here that describes how to install the workaround to an existing sigil installation.
https://www.mobileread.com/forums/sho...d.php?t=277628