From time to time, while facts are being inserted into session and persisted in database, the following exception occurs.
The session is no more updated because SQL sentences can't be executed anymore. Sometimes, when session is restored from marshalled snapshot the same error happens.

2013-09-17 11:17:03,959 ERROR [org.drools.persistence.jpa.JpaTimerJobInstance] Unable to execute timer job!
java.lang.RuntimeException: Unable to load session snapshot
at org.drools.persistence.SessionMarshallingHelper.loadSnapshot(SessionMarshallingHelper.java:96)
at org.drools.persistence.SingleSessionCommandService.initKsession(SingleSessionCommandService.java:269)
at org.drools.persistence.SingleSessionCommandService.execute(SingleSessionCommandService.java:369)
at org.drools.persistence.jpa.JpaTimerJobInstance.call(JpaTimerJobInstance.java:34)
at org.drools.time.impl.DefaultTimerJobInstance.call(DefaultTimerJobInstance.java:1)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:98)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:207)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.NullPointerException
at org.drools.time.impl.JDKTimerService.removeJob(JDKTimerService.java:133)
at org.drools.common.Scheduler.removeAgendaItem(Scheduler.java:75)
at org.drools.common.DefaultAgenda.removeScheduleItem(DefaultAgenda.java:474)
at org.drools.common.DefaultAgenda.removeActivation(DefaultAgenda.java:349)
at org.drools.common.DefaultAgenda.cancelActivation(DefaultAgenda.java:687)
at org.drools.reteoo.RuleTerminalNode.retractLeftTuple(RuleTerminalNode.java:320)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateRetractLeftTuple(SingleLeftTupleSinkAdapter.java:224)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateRetractLeftTuple(SingleLeftTupleSinkAdapter.java:98)
at org.drools.reteoo.NotNode.propagateRetractLeftTuple(NotNode.java:444)
at org.drools.reteoo.NotNode.assertObject(NotNode.java:142)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:59)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:364)
at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:235)
at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:240)
at org.drools.marshalling.impl.ProtobufInputMarshaller.assertHandleIntoOTN(ProtobufInputMarshaller.java:483)
at org.drools.marshalling.impl.ProtobufInputMarshaller.readFactHandles(ProtobufInputMarshaller.java:459)
at org.drools.marshalling.impl.ProtobufInputMarshaller.readSession(ProtobufInputMarshaller.java:259)
at org.drools.marshalling.impl.ProtobufInputMarshaller.readSession(ProtobufInputMarshaller.java:116)
at org.drools.marshalling.impl.ProtobufMarshaller.unmarshall(ProtobufMarshaller.java:143)
at org.drools.persistence.SessionMarshallingHelper.loadSnapshot(SessionMarshallingHelper.java:88)
... 11 more