Errors in Phreak under heavy and multi threaded load

Several threads are started, each one starts 1 jbpm process containing rule(s) task(s).
If the threads are few, everything works fine. Under heavy load nullPointerExceptions are thown most of the time, less frequently fireAllRules never ends and CPU remains at 100%.

Aparently the setFocus method used by rule tasks is related to the problem.

The most comon error is:
Caused by: java.lang.NullPointerException
at org.drools.core.common.LeftTupleSetsImpl.removeInsert(LeftTupleSetsImpl.java:141)
at org.drools.core.common.LeftTupleSetsImpl.addDelete(LeftTupleSetsImpl.java:80)
at org.drools.core.reteoo.LeftInputAdapterNode.doDeleteSegmentMemory(LeftInputAdapterNode.java:295)
at org.drools.core.reteoo.LeftInputAdapterNode.doDeleteObject(LeftInputAdapterNode.java:266)
at org.drools.core.reteoo.LeftInputAdapterNode.retractLeftTuple(LeftInputAdapterNode.java:361)
at org.drools.core.reteoo.ObjectTypeNode.doRetractObject(ObjectTypeNode.java:334)
at org.drools.core.reteoo.ObjectTypeNode.retractObject(ObjectTypeNode.java:317)
at org.drools.core.reteoo.EntryPointNode.propagateRetract(EntryPointNode.java:358)
at org.drools.core.phreak.PropagationEntry$Delete.execute(PropagationEntry.java:172)
at org.drools.core.phreak.SynchronizedPropagationList.flush(SynchronizedPropagationList.java:96)
at org.drools.core.phreak.SynchronizedPropagationList.flush(SynchronizedPropagationList.java:69)
at org.drools.core.impl.StatefulKnowledgeSessionImpl.flushPropagations(StatefulKnowledgeSessionImpl.java:1993)