MSBuild x86 - Silverlight 4 SDK not installed

My build server won't compile by Windows Phone 7 application giving the error "The Silverlight 4 SDK is not installed".

1. The SDK is installed and the registry entries exist under HKEY_LOCAL_MACHINE\Software\Microsft\Microsoft SDKs\Sliverlight.
2. It's 开发者_开发问答only a 32-bit machine anyway, but I've tried the /p:Platform=x86 solution without result.

Just a stupid question but did the WP7 sdk is installed ? Do you have this directoy on your build server:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\Silverlight\v4.0\Profile\WindowsPhone\

with the dlls like System, System.Net etc ?