find that line in your file
<!--<startup> <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/></startup>-->
and replace that line with:
<startup useLegacyV2RuntimeActivationPolicy="true"> <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/> </startup>
just before the configuration end tag like </configuration>