Our official issue about this is here:
https://github.com/Sigil-Ebook/Sigil/issues/232
libxml2 versions >= 2.9.3 have the bug. libxml2-2.9.2 works. If you build your own, there is a patch near the end of that issue (not the first patch) that will apply over the latest libxml2-2.9.4 and even work with libxml2 master. That way you can have all of the recent bug fixes of the newer versions as well.
KevinH