ZipException when running FindBugs Ant task

Hi,

When I try running FindBugs from an Ant task I get a ZipException on my 
output file.
As you can see below, I run the task twice. The first time everything 
goes ok but the second time it fails and I don't understand why.

Can you help me? Thanks.

Yours,
Nick.


Build file:
---------
<project name="NoKeos Quality Control Reporting" default="all">
   <property name="lib.dir" location="lib" />
   <property name="plugin.dir" location="plugins"/>
   <property name="source.dir" location="src"/>
   <property name="report.dir" location="report"/>
   <property name="build.dir" location="bin"/>
   <tstamp>
       <format property="timestamp" pattern="yyyy.MM.dd-HH'h'mm'm'" />
   </tstamp>
   <property name="findbugs.dir" location="c:/java/findbugs-1.2.1" />
   <property name="findbugs.lib.dir" location="${findbugs.dir}/lib" />
   <property name="findbugs.jvmargs"
             value="-server -Xss1m -Xmx800m -Duser.language=en 
-Duser.region=EN -Dfindbugs.home=${findbugs.lib.dir}"
   />
   <property name="findbugs.xml.core.file" 
value="${report.dir}/findbugs-core-${timestamp}.xml" />
   <property name="findbugs.xml.plugins.file" 
value="${report.dir}/findbugs-plugins-${timestamp}.xml" />
   <property name="findbugs.html.file" 
value="${report.dir}/findbugs.html" />

   <!-- findbugs task definition -->
   <path id="findbugs.lib">
       <fileset dir="${findbugs.lib.dir}">
           <include name="findbugs-ant.jar" />
       </fileset>
   </path>
   <taskdef name="findbugs" 
classname="edu.umd.cs.findbugs.anttask.FindBugsTask">
       <classpath refid="findbugs.lib" />
   </taskdef>

   <target name="all" description="Runs all QC reporting tasks" 
depends="init, checkstyle-report, findbugs-report">
   </target>

   <target name="init"></target>

   <path id="client.lib">
       <fileset dir="${lib.dir}">
           <include name="client/**/*.jar" />
           <include name="common/**/*.jar" />
       </fileset>
   </path>
   <path id="plugins.lib">
       <fileset dir="${plugin.dir}">
           <include name="**/*.jar" />
           <exclude name="**/*-client.jar" />
           <exclude name="**/*-common.jar" />
           <exclude name="**/*-server.jar" />
       </fileset>
   </path>
   <path id="client.src">
       <fileset dir="${source.dir}">
           <include name="**/*.java" />
           <exclude name="**/interfaces/*.java" />
       </fileset>
   </path>
   <path id="plugins.src">
       <fileset dir="${plugin.dir}">
           <include name="**/src/**/.java" />
           <exclude name="**/interfaces/*.java" />
       </fileset>
   </path>

   <target name="findbugs-report" description="Create the FindBugs report">
       <antcall target="findbugs-analyze" />
   </target>

   <target name="findbugs-analyze">
       <!-- splitting tasks to avoid memory problems -->
       <echo level="info">Finding bugs in core classes...</echo>
       <findbugs jvmargs="${findbugs.jvmargs}"
                 home="${findbugs.dir}"
                 reportlevel="medium"
                 output="xml:withMessages"
                 outputfile="${findbugs.xml.core.file}"
                 effort="default"
                 debug="false"
       >
           <class location="${build.dir}" />
           <auxclasspath refid="client.lib" />
           <sourcepath>
               <path refid="client.src" />
           </sourcepath>
       </findbugs>
       <echo level="info">Finding bugs in plug-in classes...</echo>
       <findbugs jvmargs="${findbugs.jvmargs}"
                 home="${findbugs.dir}"
                 reportlevel="medium"
                 output="xml:withMessages"
                 outputfile="${findbugs.xml.plugins.file}"
                 effort="default"
                 debug="false"
       >
           <class location="${plugin.dir}" />
           <auxclasspath refid="client.lib" />
           <auxclasspath refid="plugins.lib" />
           <sourcepath>
               <path refid="plugins.src" />
           </sourcepath>
       </findbugs>
   </target>
</project>


Console output:
----------------
Buildfile: C:\projects\nokeos\qc-build.xml
init:
findbugs-report:
findbugs-analyze:
    [echo] Finding bugs in core classes...
[findbugs] Running FindBugs...
[findbugs] The following classes needed for analysis were missing:
[findbugs]   javax.servlet.http.HttpServlet
   [...]
[findbugs]   org.dom4j.io.DOMReader
[findbugs] Warnings generated: 304
[findbugs] Missing classes: 33
[findbugs] Classes needed for analysis were missing
[findbugs] Output saved to 
C:\projects\nokeos\report/findbugs-core-2007.09.21-11h01m.xml
    [echo] Finding bugs in plug-in classes...
[findbugs] Running FindBugs...
[findbugs] Exception in thread "main" java.util.zip.ZipException: Error 
opening C:\projects\nokeos\report\findbugs-plugins-2007.09.21-11h01m.xml
[findbugs]     at 
edu.umd.cs.findbugs.classfile.impl.ZipFileCodeBase.<init>(ZipFileCodeBase.java:61)
[findbugs]     at 
edu.umd.cs.findbugs.classfile.impl.ClassFactory.createFilesystemCodeBase(ClassFactory.java:96)
[findbugs]     at 
edu.umd.cs.findbugs.classfile.impl.FilesystemCodeBaseLocator.openCodeBase(FilesystemCodeBaseLocator.java:63)
[findbugs]     at 
edu.umd.cs.findbugs.classfile.impl.ClassPathBuilder.processWorkList(ClassPathBuilder.java:381)
[findbugs]     at 
edu.umd.cs.findbugs.classfile.impl.ClassPathBuilder.build(ClassPathBuilder.java:192)
[findbugs]     at 
edu.umd.cs.findbugs.FindBugs2.buildClassPath(FindBugs2.java:432)
[findbugs]     at edu.umd.cs.findbugs.FindBugs2.execute(FindBugs2.java:160)
[findbugs]     at edu.umd.cs.findbugs.FindBugs.runMain(FindBugs.java:1521)
[findbugs]     at edu.umd.cs.findbugs.FindBugs2.main(FindBugs2.java:731)
[findbugs] Output saved to 
C:\projects\nokeos\report/findbugs-plugins-2007.09.21-11h01m.xml
all:
BUILD SUCCESSFUL
Total time: 2 minutes 34 seconds