[DiapDealer]

Whatever you're using to freeze/compile your Python program may not be properly including/handling the necessary Qt dependencies/locations. Python compilers typically require special configs to deal with (Py)Qt.

From your description, it's hard to tell whether it's your compiled Python program that's failing to find Qt, or Sigil itself. The fact that you say everything works without freezing your python program suggests to me that it's the former.

It's also possible that the freezing involves setting certain Qt runtime environment variables that could definitely break Sigil's ability to find it's own Qt elements.

Using a compiled Python/Qt program to launch another C++/Qt program via a subprocess is always going to be fraught with peril (because of the environment the subprocess inherits). You're probably going to have to find a way to sanitize the environment used by the subprocess launching Sigil.