Ok, finally got it.
Change this line in %windir%\System32\inetsrv\Config\ApplicationHost.config
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />
To
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler,runtimeVersionv2.0" />
If this is not enough
Add this following line to the Web.config
<system.webServer>
<modules runAllManagedModulesForAllRequests="true"/>
</system.webServer>