multiple users overwrite existing log file when RollingFileAppender is rolling over date and minimal locking is used

config settings for rolling file appender

<?xml version="1.0" encoding="utf-8" ?>
<log4net debug="false">
<appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender,log4net">
<param name="File" value="C:\\Projects\\DealerMailer\\DealerMailer.UI\\DealerMailer.UI\\Log
DealerMailer.UI_" />
<param name="RollingStyle" value="Date" />
<param name="DatePattern" value="ddMMyyyy.lo\g" />
<param name="StaticLogFileName" value="false" />
<param name="AppendToFile" value="true" />
<encoding value="utf-8" />
<param name="ImmediateFlush" value="true" />
<param name="LockingModel" type="log4net.Appender.FileAppender+MinimalLock" />
<layout type="log4net.Layout.PatternLayout,log4net">
<param name="ConversionPattern" value="%d [%-5level] [%logger] - %m%n" />
</layout>
</appender>

<logger name="DealerMailerUI">
<priority value="ALL" />
<appender-ref ref="RollingFileAppender" />
</logger>

</log4net>

problem
today 070702009
user1 logs into application today --> logs data in log file dated today (DealerMailer.UI_07072009.log)
user2 logs into application today --> logs data in log file dated today (DealerMailer.UI_07072009.log)
user2 logs out of application today --> logs data in log file dated today (DealerMailer.UI_07072009.log)

tomorrow 08072009
user1 still has his application open from yesterday (all calls are per call basis so no locks are held)
user2 logs into application --> no file exists, rolls over to next day & logs data (DealerMailer.UI_08072009.log)
user1 submits a transaction --> all data in (DealerMailer.UI_08072009.log) is overwritten

i have no idea why this is happening