The area of sigil_bs4 that's failing is in a section of bs4 that is intended as a workaround to a Python bug that was fixed in 2013:
https://github.com/Sigil-Ebook/Sigil...lparser.py#L74
I'm just not certain why it never caused problems on any official (non-legacy) versions of Sigil before 1.9.x on Windows 7.