NPE with Perf4j and Log4j

while using Perf4j together with Log4j I get the following exception:

log4j:ERROR Error occured while converting date.
java.lang.NullPointerException
at java.lang.System.arraycopy(Native Method)
at java.lang.AbstractStringBuilder.getChars(AbstractStringBuilder.java:328)
at java.lang.StringBuffer.getChars(StringBuffer.java:201)
at org.apache.log4j.helpers.ISO8601DateFormat.format(ISO8601DateFormat.java:130)
at java.text.DateFormat.format(DateFormat.java:316)
at org.apache.log4j.helpers.PatternParser$DatePatternConverter.convert(PatternParser.java:444)
at org.apache.log4j.helpers.PatternConverter.format(PatternConverter.java:65)
at org.apache.log4j.PatternLayout.format(PatternLayout.java:502)
at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:302)
at org.apache.log4j.WriterAppender.append(WriterAppender.java:160)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.perf4j.log4j.AsyncCoalescingStatisticsAppender$1.handle(AsyncCoalescingStatisticsAppender.java:200)
at org.perf4j.helpers.GenericAsyncCoalescingStatisticsAppender$Dispatcher.run(GenericAsyncCoalescingStatisticsAppender.java:316)
at java.lang.Thread.run(Thread.java:662)

My log4j.xml looks like this:

<!-- Appenders -->
<appender name="console" class="org.apache.log4j.ConsoleAppender">
    <param name="Target" value="System.out" />
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="%d %-5p: %c - %m%n" />
    </layout>
</appender>

<!-- Perf4j Appender -->
<appender name="CoalescingStatistics"
          class="org.perf4j.log4j.AsyncCoalescingStatisticsAppender">
    <param name="TimeSlice" value="10000"/>
    <appender-ref ref="fileAppender"/>
</appender>

<!-- This file appender is used to output aggregated performance statistics -->
<appender name="fileAppender" class="org.apache.log4j.FileAppender">
    <param name="File" value="perfStats.log"/>
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="%m%n"/>
    </layout>
</appender>

<!-- Perf4j Loggers -->
<logger name="org.perf4j.TimingLogger" additivity="false">
    <level value="INFO"/>
    <appender-ref ref="CoalescingStatistics"/>
</logger>

What am I doing wrong?

I hope I can help someone who just ran into the same exception. So here is my answer:

It seems that the described exception only appears under a few circumstances. Here is my setup:

Developing a web application
Tomcat 6.0.32
Log4j 1.2.15
Perf4j 0.9.16

Furthermore it seems that the exception only occurs after one redeploys the web application. The exception does NOT occur if you just fresh start the tomcat.

What I have done to get rid of the exception: