Hi Henk,
Are you using a proxy server on your computer?
To access the Windows Store, to download and run apps, you need an active Internet connection and a screen resolution of at least 1024 x 768.
Refer to these methods in the order in which they are presented and check.
Method 1:
I would suggest you to refer to the article and check if it helps. Ignore the steps you have tried.
What to do if you have problems with an app
http://windows.microsoft.com/en-us/windows-8/what-troubleshoot-problems-app
Method 2:
If you are running the Microsoft Account, run the Microsoft Troubleshooter to check if Microsoft Account is working fine *Microsoft Accounts Troubleshooter* is designed to deal the issues related with Microsoft accounts. When you run it, it will automatically detect common issues.
Method 3: If you are using proxy connection, please disable proxy.
Follow the steps to disable proxy:
a. Click the Gearbox at the top right corner in internet explorer.
b. Click the Tools button, and then click Internet Options.
c. Click the Connections tab, and then click LAN settings.
d. Uncheck the box next to “proxy server for your LAN”.
Method 4: Run the System File Checker tool and check.
System File Checker.
http://support.microsoft.com/kb/929833/en-us
Hope this information helps you. Please do let us know if you need further assistance, we’ll be glad to assist you.