[...]
Caused by: org.apache.excalibur.source.SourceException: Exception during processing of cocoon://_plugins/...
	at org.apache.cocoon.components.source.impl.SitemapSource.getInputStream(SitemapSource.java:277)
    [...]
	... 36 more
Caused by: java.lang.NullPointerException
	at java.util.Hashtable.put(Hashtable.java:394)
	at org.apache.excalibur.store.impl.MRUMemoryStore.hold(MRUMemoryStore.java:216)
	at org.apache.excalibur.store.impl.MRUMemoryStore.store(MRUMemoryStore.java:190)
	at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.generateLock(AbstractCachingProcessingPipeline.java:255)
	at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.processXMLPipeline(AbstractCachingProcessingPipeline.java:337)
	at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.process(AbstractProcessingPipeline.java:483)
	at org.apache.cocoon.components.source.impl.SitemapSource.getInputStream(SitemapSource.java:267)
	... 37 more