我用的是log4net 1.2.11下面是我使用的示例配置（我使用的C#代码与您的相同）：

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

  <log4net>
    <appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
      <file type="log4net.Util.PatternString" value="default_%property{fname}.log"/>
      <appendToFile value="true"/>
      <rollingStyle value="Size"/>
      <maxSizeRollBackups value="10"/>
      <maximumFileSize value="100KB"/>
      <staticLogFileName value="true"/>
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%date [%thread] %-5level %logger [%property{NDC}] - %message%newline"/>
      </layout>
    </appender>

    <root>
      <priority value="ALL"/>
      <appender-ref ref="RollingFileAppender"/>
    </root>

  </log4net>
</configuration>

这个应该可以
我使用的是log4net 1.2.13.0。您应该考虑的一件事是为log4net添加一个内部调试，它将告诉您实际的错误。

class Program
    {
        private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
        static void Main(string[] args)
        {
            log4net.GlobalContext.Properties["fname"] = "aaaa";

            log4net.Config.XmlConfigurator.Configure();
            log.Debug("Test");
        }
    }

配置部分

<configuration>      
  <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net"/>
  </configSections>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2" />
  </startup>
  <log4net debug="true">
    <appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
      <file type="log4net.Util.PatternString" value="D:\default_%property{fname}.log"/>
      <appendToFile value="true"/>
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%date [%thread] %-5level %logger - %message%newline"/>
      </layout>
    </appender>    
    <root>
      <priority value="ALL"/>
      <appender-ref ref="RollingFileAppender"/>
    </root>    
  </log4net>
  <system.diagnostics>
    <trace autoflush="true">
      <listeners>
        <add
            name="textWriterTraceListener"
            type="System.Diagnostics.TextWriterTraceListener"
            initializeData="E:\USERS\vivek.meka\Documents\Visual Studio 2015\Projects\log4net.txt" />
      </listeners>
    </trace>
  </system.diagnostics>
</configuration>