Jip, tried that. Also removed the QTDIR variable and replaced it with the links itself to the right Qt version + folder. :)
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Take the pro file that *does* work in release when built by Creator, and use MSVS + Qt Addin to make a vcproj from it. Run it and... what happens? Make sure to to a full clean before building.
Release version... Works!!!! :-)
I created an entire new project and now it works fine!
Thank you guys for your help!!