Plus, after building a release version on MacOS, run the macdeployqt tool to have all Qt dependencies inside the app.

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That's very promising indeed! Thank you very much! have a nice day! Cheers Manfred

I was able to resolve this problem. The issue was, I had run "nmake install" instead of "nmake". refer to : https://bugreports.qt.io/browse/QTBUG-44862.