I had the same problem.
I used
<fileset dir="${basedir}/build">
<include name="**/*.class"/>
</fileset>
inside findbugs target and it seems that there is too much .class files to be passed to findbug (?via command line?) because when I used
<fileset dir="${basedir}/build/com/domain/package">
<include name="**/*.class"/>
</fileset>
that had low number of classes, the error was gone.
So, I solved the problem by making one jar file and feeding it to findbugs target with
<findbugs home="${findbugs.home}">
...
<class location="${basedir}/targets/classes-to-analyze.jar"/>
</findbugs>
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