Unable to remove component from automatic release: no environment available.

[Bj�rn Voigt]

Hello cocoondevs,
we are running cocoon-2.1.5 app and after around 150.000 requests
we get the following error in our error.log and the app. generates no longer
content must be restarted.
Have you any idea about possible reasons or a solution,
which could help us to isolate the problem.
org.apache.cocoon.ProcessingException: Unable to remove component from 
automatic release: no environment available.
        at 
org.apache.cocoon.components.CocoonComponentManager.removeFromAutomaticRelease(CocoonComponentManager.java:448)
        at 
org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.release(AbstractProcessingPipeline.java:173)
        at 
org.apache.cocoon.components.source.impl.SitemapSource.reset(SitemapSource.java:454)
        at 
org.apache.cocoon.components.source.impl.SitemapSource.recycle(SitemapSource.java:477)
        at 
org.apache.cocoon.components.source.impl.SitemapSourceFactory.release(SitemapSourceFactory.java:78)
        at 
org.apache.excalibur.source.impl.SourceResolverImpl.release(SourceResolverImpl.java:269)
        at 
org.apache.cocoon.components.CocoonComponentManager.release(CocoonComponentManager.java:507)
        at 
org.apache.cocoon.components.CocoonComponentManager.release(CocoonComponentManager.java:507)
        at 
org.apache.cocoon.components.CocoonComponentManager.release(CocoonComponentManager.java:507)
        at 
org.apache.cocoon.environment.AbstractEnvironment.release(AbstractEnvironment.java:539)
        at 
org.apache.cocoon.environment.wrapper.MutableEnvironmentFacade.release(MutableEnvironmentFacade.java:320)
        at 
org.apache.cocoon.sitemap.ContentAggregator.recycle(ContentAggregator.java:268)
        at 
org.apache.avalon.excalibur.pool.ResourceLimitingPool.put(ResourceLimitingPool.java:438)
        at 
org.apache.avalon.excalibur.component.PoolableComponentHandler.doPut(PoolableComponentHandler.java:212)
        at 
org.apache.avalon.excalibur.component.ComponentHandler.put(ComponentHandler.java:425)
[...]

Thank you
Bj�rn Voigt