Can't update to Lion: Serious Mac App Store problem

Question

I tried twice to update my OS (10.6.8) to Lion via the Mac App Store. Both times I got a failure message shortly before I had downloaded the 3,74 GB, saying the app can't be downloaded and that I have to restart the downloading (original message in German: "Die App kann nicht geladen werden. Das Laden des gewünschten Programms ist fehlgeschlagen. Bitte versuchen sie es erneut"). 

Answer 1

You must sign in to the App Store with the Apple ID you used to install the app. If you don't recognize the ID, see below.
You installed a hacked app, originally from the Mac App Store. It contains the receipt for a different app, downloaded using an account that you don't control. You need to identify and remove the hacked app.
Important: The app you need to remove is not necessarily the one named in the App Store notice. For example, if the App Store says you need to update "Twitter," the hacked app may be "Angry Birds" or something else entirely. Don't make any assumptions about which app you're looking for. To find it, you have to carry out a systematic search.
Triple-click anywhere in the line of text below on this page to select it:
kMDItemAppStoreHasReceipt=1
Copy the selected text to the Clipboard (command-C).
In the Finder, press the key combination command-F to open a search window, or select
File ▹ Find
from the menu bar. In the search window, select
Search: This Mac
from the row of tokens below the toolbar. Below that is a popup menu initially showing Kind. From that menu, select  Other...
A sheet will drop down. In that sheet, select Raw Query and click OK or press return.
Now there will be a text box to the right of popup menu. Click in that box and paste (command-V).
The search window will show all the App Store products you've installed. Compare those search results with the list of your purchases from the App Store. To see the complete list, you may need to unhide hidden purchases. If any apps were download from the App Store using other Apple ID accounts that you control, sign in to the store under each of those ID's and check the purchases.
At least one of the items listed in the search window is not among your purchases in the App Store. Move each such item to the Trash. You may be prompted for your administrator password. Empty the Trash.
Log out and log back in. Test.