NullPointerException halting jBPM process execution

Issue

Some of our jBPM process instances are stopped in the middle of execution. In the server.log file we see the following ERROR:

ERROR (ExceptionHandler) exception thrown in process process1 in transition to to_node3 - halting this process
java.lang.NullPointerException
 at java.io.ByteArrayInputStream.<init>(ByteArrayInputStream.java:89)
 at org.jbpm.context.exe.converter.SerializableToByteArrayConverter.revert(SerializableToByteArrayConverter.java:81)
 at org.jbpm.context.exe.VariableInstance.getValue(VariableInstance.java:132)
 at org.jbpm.context.exe.VariableContainer.getVariablesLocally(VariableContainer.java:116)
 at org.jbpm.context.exe.VariableContainer.getVariables(VariableContainer.java:101)
 at org.jbpm.context.exe.ContextInstance.getVariables(ContextInstance.java:107)
 at org.jbpm.graph.action.Script.createInputMap(Script.java:97)
 at org.jbpm.graph.action.Script.eval(Script.java:78)
 at org.jbpm.graph.action.Script.execute(Script.java:65)
 at org.jbpm.graph.def.GraphElement.executeActionImpl(GraphElement.java:300)
 at org.jbpm.graph.def.GraphElement.executeAction(GraphElement.java:267)
 at org.jbpm.graph.def.GraphElement.executeActions(GraphElement.java:240)
 at org.jbpm.graph.def.GraphElement.fireAndPropagateEvent(GraphElement.java:206)
 at org.jbpm.graph.def.GraphElement.fireEvent(GraphElement.java:188)
 at org.jbpm.graph.def.Transition.take(Transition.java:150)
 at org.jbpm.graph.def.Node.leave(Node.java:450)
 ...