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MacBook Pro, Mac OS X (10.7.4)
Installing the OS X Lion Update 10.7.4 (Client Combo) reinstalls the App Store which may help.
Restart your Mac after the combo is installed.
It's ok to do this even though you are already running v10.7.4.
Then try downloading Mountain Lion.
Thanks a lot Carolyn for the tips, but i didn't manage to get it working through reinstallation of the 10.7.4 combo.
Deb from iTunes Store customer support pointed me to this beauitful article,
https://discussions.apple.com/thread/2710174?answerId=16063398022#16063398022
I didn't have to install any package, just delete the corrupted file and it works brilliantly after that.
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