Preview is mainly QtWebEngine so a few places to start:
1. Try updating Your video driver to the latest version
2. Try making sure no “anti-virus” software is interfering
3. Try booting into safe-mode to test if some other kernel extension is causing the problem
4. Try launching Sigil from Terminal.app to look for any debug error messages
5. Try running qtdiag downloaded from Qt to see what it says
6. Try setting some of the Qt environment variables described here and launching Sigil via Terminal.app to see if any help.
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