MainWindow Menu bug in Qt 4.2.3?

[No-Nonsense]

I switched to Qt 4.2.3 from Qt 4.2.2 yesterday and my Qt Designer build QMainWindow application now has a severe bug with the menu. I attached two screenshots showing the problem with both languages my app supports:
There is a small line above the menu. Below this line the menu items are not clickable. As a workaround to access File / Edit you have to click a menu item that is on the right from that line and then move the mouse left so that the menu switches to the leftnext menu.

Previewing the MainWindow in Qt Designer does not show this line and behaviour. Another app and QDevelop that I both build with Qt 4.2.3 do not show this either.

Any ideas what could cause this issue?

Thanks in advance,
-Jens