load generator error while reaching 39 users ?

I am getting the below error while reaching 39 users in the stress test.

2020/04/27 14:43:08 INFO - neoload.LG_INFO: Starting LoadGenerator Version 6.9.0 ;build=20190307-5
2020/04/27 14:43:08 INFO - neoload.LG_INFO: OS Version: x86 - Windows 10 - 10.0
2020/04/27 14:43:08 INFO - neoload.LG_INFO: JVM Version: Oracle Corporation - 1.8.0_112 - Java HotSpot(TM) Server VM - Xmx= 745799680 bytes
2020/04/27 14:43:08 INFO - neoload.LG_INFO: Sockets Tuning information: MaxUserPort=65534; TcpTimedWaitDelay=30;
2020/04/27 14:43:08 INFO - neoload.LG_INFO: ALPN boot status: present
2020/04/27 14:43:09 INFO - neoload.LG: Registered to Agent. Port 37098
2020/04/27 14:43:09 INFO - neoload.LG: Launching inbound connection listener on port:59280
2020/04/27 14:43:09 INFO - neoload.LG: Connection established with the Controller localhost:7100:59280
2020/04/27 14:43:10 INFO - neoload.LG_INFO: WAN Emulation: disabled
2020/04/27 14:43:12 INFO - neoload.LG: Connection established with the Controller localhost:7100:59280
2020/04/27 14:43:12 INFO - neoload.LG_INFO: Jetty ThreadPool configuration: MinThread=20; MaxThread=20000; IdleTimeoutMs=10000; QueueSize=6000
2020/04/27 14:47:37 ERROR - neoload.Engine: Error during variable modification NL-STOP-TEST
at com.neotys.nl.controller.data.variables.woov.StopTestStrategy.apply(StopTestStrategy.java:44)
at com.neotys.nl.controller.data.variables.AbstractVariable.handleOutOfValues(AbstractVariable.java:418)
at com.neotys.nl.controller.data.variables.unicity.NextValueStrategy.changeValue(NextValueStrategy.java:26)
at com.neotys.nl.controller.data.variables.multi.FileVariable.delegateChangeValue(FileVariable.java:200)
at com.neotys.nl.controller.data.variables.AbstractVariable.changeValue(AbstractVariable.java:129)
at com.neotys.nl.controller.data.variables.SelfVariableChangeImpl.changeValue(SelfVariableChangeImpl.java:86)
at com.neotys.nl.controller.data.variables.SelfVariableChangeImpl.getNext(SelfVariableChangeImpl.java:75)
at com.neotys.nl.controller.data.variables.AbstractVariable.getNext(AbstractVariable.java:175)
at com.neotys.nl.agent.transport.AgentServiceLgReverseImpl.sendNeoLoadRequestToController(AgentServiceLgReverseImpl.java:174)
at sun.reflect.GeneratedMethodAccessor1175.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.receivedRequest(EndPointsRegistryImpl.java:170)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.handleDataReceived(EndPointsRegistryImpl.java:125)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.dataReceived(EndPointsRegistryImpl.java:113)
at com.neotys.communication.reliable.transport.client.ReliableClientListener.dataReceived(ReliableClientListener.java:38)
at com.neotys.communication.transport.fragments.FragmentListener.dataReceived(FragmentListener.java:47)
at com.neotys.communication.transport.listener.ThreadPoolListenerExecutor$1.run(ThreadPoolListenerExecutor.java:47)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

2020/04/27 14:47:37 ERROR - neoload.Engine: Error during variable modification NL-STOP-TEST
at com.neotys.nl.controller.data.variables.woov.StopTestStrategy.apply(StopTestStrategy.java:44)
at com.neotys.nl.controller.data.variables.AbstractVariable.handleOutOfValues(AbstractVariable.java:418)
at com.neotys.nl.controller.data.variables.unicity.NextValueStrategy.changeValue(NextValueStrategy.java:26)
at com.neotys.nl.controller.data.variables.multi.FileVariable.delegateChangeValue(FileVariable.java:200)
at com.neotys.nl.controller.data.variables.AbstractVariable.changeValue(AbstractVariable.java:129)
at com.neotys.nl.controller.data.variables.SelfVariableChangeImpl.changeValue(SelfVariableChangeImpl.java:86)
at com.neotys.nl.controller.data.variables.SelfVariableChangeImpl.getNext(SelfVariableChangeImpl.java:75)
at com.neotys.nl.controller.data.variables.AbstractVariable.getNext(AbstractVariable.java:175)
at com.neotys.nl.agent.transport.AgentServiceLgReverseImpl.sendNeoLoadRequestToController(AgentServiceLgReverseImpl.java:174)
at sun.reflect.GeneratedMethodAccessor1175.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.receivedRequest(EndPointsRegistryImpl.java:170)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.handleDataReceived(EndPointsRegistryImpl.java:125)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.dataReceived(EndPointsRegistryImpl.java:113)
at com.neotys.communication.reliable.transport.client.ReliableClientListener.dataReceived(ReliableClientListener.java:38)
at com.neotys.communication.transport.fragments.FragmentListener.dataReceived(FragmentListener.java:47)
at com.neotys.communication.transport.listener.ThreadPoolListenerExecutor$1.run(ThreadPoolListenerExecutor.java:47)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

2020/04/27 14:47:37 ERROR - neoload.Engine: Error during variable modification NL-STOP-TEST
at com.neotys.nl.controller.data.variables.woov.StopTestStrategy.apply(StopTestStrategy.java:44)
at com.neotys.nl.controller.data.variables.AbstractVariable.handleOutOfValues(AbstractVariable.java:418)
at com.neotys.nl.controller.data.variables.unicity.NextValueStrategy.changeValue(NextValueStrategy.java:26)
at com.neotys.nl.controller.data.variables.multi.FileVariable.delegateChangeValue(FileVariable.java:200)
at com.neotys.nl.controller.data.variables.AbstractVariable.changeValue(AbstractVariable.java:129)
at com.neotys.nl.controller.data.variables.SelfVariableChangeImpl.changeValue(SelfVariableChangeImpl.java:86)
at com.neotys.nl.controller.data.variables.SelfVariableChangeImpl.getNext(SelfVariableChangeImpl.java:75)
at com.neotys.nl.controller.data.variables.AbstractVariable.getNext(AbstractVariable.java:175)
at com.neotys.nl.agent.transport.AgentServiceLgReverseImpl.sendNeoLoadRequestToController(AgentServiceLgReverseImpl.java:174)
at sun.reflect.GeneratedMethodAccessor1175.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.receivedRequest(EndPointsRegistryImpl.java:170)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.handleDataReceived(EndPointsRegistryImpl.java:125)
at com.neotys.communication.endpoint.EndPointsRegistryImpl.dataReceived(EndPointsRegistryImpl.java:113)
at com.neotys.communication.reliable.transport.client.ReliableClientListener.dataReceived(ReliableClientListener.java:38)
at com.neotys.communication.transport.fragments.FragmentListener.dataReceived(FragmentListener.java:47)
at com.neotys.communication.transport.listener.ThreadPoolListenerExecutor$1.run(ThreadPoolListenerExecutor.java:47)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)