After updating my Windows 10 Technical Preview to the latest build published earlier (build 9926), all App Store (the current one and the new one in beta) were not working to get any new application and where stuck in loading as shown in the screenshot below

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The solution was to run the following commands using an elevated command prompt

powershell -ExecutionPolicy Unrestricted Add-AppxPackage -DisableDevelopmentMode -Register $Env:SystemRoot\WinStore\AppxManifest.xml

start "" "ms-windows-store:"

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Then restart you client, et voila

Thank you to Daniel Dixon – http://blogs.technet.com/b/ouc1too/archive/2013/10/19/windows-app-store-doesn-t-load-after-8-1-upgrade.aspx