@jakub.f the add-on is suppressing backend errors, which is resulting here in blank cards and incomplete rendering of the review screen. Specifically, the issue here is that the following TTS-related error is thrown:
anki.errors.BackendError: WindowsError { details: Other, source: Error { code: HRESULT(0x80070002), message: "The system cannot find the file specified." } }
The add-on is logging that in _try_handle_exception() and some routine is probably failing to propagate it.