Esta é minha mensagem de erro:

log4net:ERROR XmlConfigurator: Failed to find configuration section 'log4net' in the application's .config file. Check your .config file for the <log4net> and <configSections> elements. The configuration section should look like: <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net" />

este é o meu web.config:

<?xml version="1.0"?>
<configuration>
    <configSections>
       <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
    </configSections>

   <system.serviceModel>
   ...
   </system.serviceModel>

   <connectionStrings>
   ...
   </connectionStrings>

   <log4net>
   ...
   </log4net>

</configuration>

O que há de errado com minha configuração?

Atualizar:

Tem também Web.Release.config:

<?xml version="1.0"?>    
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">

    <system.web>
        <compilation xdt:Transform="RemoveAttributes(debug)" />
    </system.web>

    <system.serviceModel>
    ...
    </system.serviceModel>

    <connectionStrings>
    ...
    </connectionStrings>

    <log4net>
    ...   
       <root>
          <level value="DEBUG" xdt:Transform ="Replace"/>
       </root>
    </log4net>

</configuration>

Web.Test.cofig - o mesmo que o Release one

e Web.Debug.config, que está vazio:

<?xml version="1.0"?>

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">    

</configuration>