Hello,
Please post (or send to contact[at]phpdocx.com) the most simple code that illustrates your issue and if you are using the classic or namespaces package, so we can test it. Also post the full output of DOMNode:: insertBefore(): Document Fragment is empty (including the line that returns the warning).
The problem seems you are mixing HTML that contains custom list styles and standard lists, so you may get a warning when trying to mix them in the same embedHTML call. The next release of phpdocx will include a minor check to prevent this warning when using custom list styles and standard lists in the same HTML; if the line is the same we'll post it in this same thread so you can apply it to your current version.
Regards.