Finally , I have to use the different mirror that the default as listed in: https://download.qt.io/online/qtsdkrepository/windows_x86/root/qt/Updates.xml.mirrorlist
after browsing here and there , I must put the mirror without full path to xml. Instead only url before the /online part.
qt-online-installer-windows-x64-4.8.1.exe --mirror https://qt-mirror.dannhauer.de/
and voila!
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