Hi Thomas ...
If you have anti virus software installed, disable that.
Try turning off the Firewall in System Preferences > Security & Privacy
If you have an app called Little Snitch installed, that can prevent apps from downloading.
Try deleting the cache associated with the App Store.
Quit the App Store if it's open.
Go to ~/Library/Caches/com.apple.appstore/Cache.db
Move the Cache.db file from the com.apple.appstore folder to the Trash.
For Lion v10.7x: To find the Home folder in OS X Lion, open the Finder, hold the Option key, and choose Go > Library
Relaunch the App Store to test.
edited by: cs
Installing the OS X Lion Update 10.7.3 (Client Combo) will reinstall the App Store for you which may help. It's ok to do this even though you are already running v10.7.3.
Restart your Mac after the combo is installed.
Try the App Store.
thank you very much for your patience but I'm afraid I still get the same old error code.
But this morning I noticed that the price tag has changed overnight. Yesterday when I bought the programme it cost € 2.99 and today it's priced at € 23.99
I'm pretty sure it's got something to do with that.