System Error when running loadUi()

Hello,
I'm pretty new to Qt, Python and their combinations. I'm currently writing a QGIS plugin in python (I used QtCreator 2.1 (Qt Designer 4.7) to generate a .ui-file and am now trying to use it for a Quantum GIS plugin that's written in Python 2.5 (and running in the Quantum GIS Python 2.5 console)).
I am running into trouble when loading the ui-file dynamically when the program runs the loadUi() function. The error occurs outside my script. Does that mean, I'm passing something wrong into it? Where does the error come in? Any hints on what could be wrong?

Code:

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# Set up the user interface from Designer
code_dir = os.path.dirname(os.path.abspath(__file__))
self.ui = loadUi(os.path.join(code_dir, "Ui_myfile.ui"), self)

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Quote:

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Traceback (most recent call last):
File "C:/Dokumente und Einstellungen/name.name/.qgis/python/plugins\myfile\myfile_plugin.py", line 57, in run
self.dialog = myfileUI(self.iface)
File "C:/Dokumente und Einstellungen/name.name/.qgis/python/plugins\myfile\myfile_gui.py", line 42, in __init__
self.ui = loadUi(os.path.join(code_dir, "Ui_myfile.ui"), self)
File "C:\PROGRA~1\QUANTU~1\apps\Python25\lib\site-packages\PyQt4\uic\__init__.py", line 112, in loadUi
return DynamicUILoader().loadUi(uifile, baseinstance)
File "C:\PROGRA~1\QUANTU~1\apps\Python25\lib\site-packages\PyQt4\uic\Loader\loader.py", line 21, in loadUi
return self.parse(filename)
File "C:\PROGRA~1\QUANTU~1\apps\Python25\lib\site-packages\PyQt4\uic\uiparser.py", line 768, in parse
actor(elem)
File "C:\PROGRA~1\QUANTU~1\apps\Python25\lib\site-packages\PyQt4\uic\uiparser.py", line 616, in createUserInterface
self.traverseWidgetTree(elem)
File "C:\PROGRA~1\QUANTU~1\apps\Python25\lib\site-packages\PyQt4\uic\uiparser.py", line 594, in traverseWidgetTree
handler(self, child)
File "C:\PROGRA~1\QUANTU~1\apps\Python25\lib\site-packages\PyQt4\uic\uiparser.py", line 233, in createWidget
topwidget.setCentralWidget(widget)
SystemError: error return without exception set

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