I changed the content of App.config. It now contains :
<enterpriselibrary.configurationsource selectedsource="File-based Configuration Source">
<sources>
<add name="File-based Configuration Source" type="Microsoft.Practices.EnterpriseLibrary.Common.Configuration.FileConfigurationSource, Microsoft.Practices.EnterpriseLibrary.Common, Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
filePath="C:\LoggingBlock.config" />
</add></sources>
</enterpriselibrary.configurationsource>
Nothing was changed in the application. :)