Log4net starts working when setting internal debugging on - asp.net

Question

I have .NET 4.5.2 Web API 2 application on IIS 8 server which uses log4net (1.2.13.0) for logging. I'm also using Windsor Castle. The problem is that log4net logging is working fine on production server for days, even weeks. But then, for unknown reason, it stops writing anything to log file. My log4net configuration is following:
<configuration>
<configSections>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
</configSections>
<log4net>
<appender name="TextFileAppender" type="log4net.Appender.RollingFileAppender">
<file value="C:\Logs\MyApplication.log" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %level %property{Guid} - %message%n" />
</layout>
</appender>
<appender name="SmtpAppender" type="log4net.Appender.SmtpAppender">
<to value="toaddress#domain.com" />
<from value="noreply#domain.com" />
<subject value="MyApplication production environment - ERROR" />
<smtpHost value="my.smtp.server.com" />
<bufferSize value="1" />
<lossy value="false" />
<evaluator type="log4net.Core.LevelEvaluator">
<threshold value="ERROR" />
</evaluator>
<filter type="log4net.Filter.LevelRangeFilter">
<levelMin value="ERROR" />
<acceptOnMatch value="true" />
</filter>
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %level %property{Guid} - %message%n" />
</layout>
</appender>
<root>
<level value="All"></level>
<appender-ref ref="TextFileAppender" />
<appender-ref ref="SmtpAppender"/>
</root>
</log4net>
</configuration>
I first tried setting write permissions to the Logs-folder for IIS accounts - without any effect. I first suspected this since this is customer's server and it's possible that some admin was messing around with folder permissions. Then I set log4net internal debugging on with following settings:
<appSettings>
<add key="log4net.Internal.Debug" value="true"/>
<appSettings>
<system.diagnostics>
<trace autoflush="true">
<listeners>
<add
name="textWriterTraceListener"
type="System.Diagnostics.TextWriterTraceListener"
initializeData="log4net.txt" />
</listeners>
</trace>
</system.diagnostics>
It writes log4net.txt file, and everything seems to be in order. Some example from the internal log:
log4net: Searched for existing files in [C:\Logs]
log4net: curSizeRollBackups starts at [0]
log4net: [.2016-05-26] vs. [.2016-05-30]
log4net: Initial roll over to [C:\Logs\MyApplication.log.2016-05-26]
log4net: Moving [C:\Logs\MyApplication.log] -> [C:\Logs\MyApplication.log.2016-05-26]
log4net: curSizeRollBackups after rollOver at [0]
log4net: Opening file for writing [C:\Logs\MyApplication.log] append [True]
log4net: Created Appender [TextFileAppender]
log4net: Adding appender named [TextFileAppender] to logger [root].
log4net: Loading Appender [SmtpAppender] type: [log4net.Appender.SmtpAppender]
As you can see, there was 4 days gap when logs were not written. But now the strange thing. The logging is now also successfully written to C:\Logs\MyApplication.log file, even when I remove settings for log4net internal debugging. So it seems that setting log4net internal debugging on fixed the problem. But this is not proper long term solution.
Any ideas what's happening here?