This is what I get in the log when hudson tries to load the japex results:

Recording japex reports <...>/reports/japex/*/report.xml
<...>/reports/japex/2009_05_19_21_00/report.xml
FATAL: null
java.lang.NullPointerException
at com.sun.japex.RegressionDetector.generateXmlReport(RegressionDetector.java:160)
at hudson.plugins.japex.JapexPublisher.perform(JapexPublisher.java:140)
at
hudson.tasks.BuildStepCompatibilityLayer.perform(BuildStepCompatibilityLayer.java:56)
at
hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:372)
at
hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:360)
at hudson.model.Build$RunnerImpl.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:345)
at hudson.model.Run.run(Run.java:947)
at hudson.model.Build.run(Build.java:112)
at hudson.model.ResourceController.execute(ResourceController.java:93)
at hudson.model.Executor.run(Executor.java:119)