I developed a kinda complex UI in Designer and load this in PySide6 using QUiLoader.
It worked during development, but just when I thought I was finished it doesn’t show anything any more. The scripts starts and runs, it just doesn’t show any window any more.

I’m running this on Windows 11, Python 3.12/PySide 6.7.2 and 3.11 with PySide.

I’ll give you a stripped down version with the same behaviour, I really hope you can help me with this:
(the path to HelloWorld.ui is correct, I dbl checked it several times)

from PySide6.QtUiTools import QUiLoader
from PySide6.QtCore import QFile
from PySide6.QtWidgets import QApplication, QWidget, QVBoxLayout

class Window(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent=parent)
        loader = QUiLoader()
        ui_file = QFile('D:/Path/to/HelloWorld.ui')
        ui_file.open(QFile.ReadOnly)
        self.window = loader.load(ui_file, self)
        root_layout = QVBoxLayout()
        root_layout.addWidget(self.window)
        self.setLayout(root_layout)
        ui_file.close()
app = QApplication.instance()
if not app:
    app = QApplication()
window = Window()
window.show()
app.exec()