Wrong colors/black rectangles on welcome screen

When running Qt Creator from the installer, there are black rectangles on the welcome page (see attached screenshot) and I get

[1996] SOFT ASSERT: "settings.contains(key)" in file C:\work\build\qtsdk\qt-creator\src\libs\utils\theme\theme.cpp, line 287

in dbgview. Everything seems to work fine, if I use another settings path.