OK. I've uploaded the test version of Sigil (it actually should be very stable) to one of my github repositories. You can download it from:
https://github.com/dougmassay/win-qt...-x64-Setup.exe
Antivirus software and/or browser security may bark at it since very few people will have downloaded it. Either you trust me or you don't.
It will be installed as Sigil 2.3.1. The installer's a little larger than the official release of Sigil 2.3.1, but only because I didn't want to wait on the ultra compression I use on releases.
If it works to solve the problem, feel free to continue to use this version until the next official release that will contain the fix.