[KevinH]

You did not say what platform you are using?

Sigil in Windows and macOS embeds its own Python interpreter and its own PySide6 version. If those two Qt versions differ then issues arise. On Linux, there is a script to set the ld library path and other sundries.

That said, passing a path to an epub as the first argument to Sigil works well on all platforms.

So why not a shell or batch file to launch Sigil passing in the path to the epub you want to edit if simply adding it on the command line is an issue.

Or try creating a python subprocess with its own shell to invoke Sigil.