After upgrade Confluence from 6.6.2 to 6.15.4 we found that
Application link from Jira > confluence  is connected
but on  confluence side shows Network error "Unexpected response"

below log could be connected with this issue
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2019-05-20 09:16:23,590 ERROR [myWorkClientChecker:thread-1] [atlassian.mywork.util.Executors] uncaughtException Uncaught exception in thread "myWorkClientChecker:thread-1": null
java.lang.NullPointerException
at com.atlassian.plugins.rest.module.jersey.JerseyEntityHandler.unmarshall(JerseyEntityHandler.java:203)
at com.atlassian.plugins.rest.module.jersey.JerseyResponse.getEntity(JerseyResponse.java:41)
at com.atlassian.mywork.host.service.ClientServiceImpl$7.handle(ClientServiceImpl.java:415)
at com.atlassian.mywork.host.service.ClientServiceImpl$7.handle(ClientServiceImpl.java:410)
at com.atlassian.plugins.rest.module.jersey.JerseyRequest$2.handle(JerseyRequest.java:134)
at com.atlassian.sal.core.net.HttpClientRequest.executeAndReturn(HttpClientRequest.java:106)
at com.atlassian.plugins.rest.module.jersey.JerseyRequest.executeAndReturn(JerseyRequest.java:131)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.atlassian.plugins.rest.module.ContextClassLoaderSwitchingProxy.invoke(ContextClassLoaderSwitchingProxy.java:22)
at com.sun.proxy.$Proxy2857.executeAndReturn(Unknown Source)
at com.atlassian.applinks.core.auth.ApplicationLinkRequestAdaptor.executeAndReturn(ApplicationLinkRequestAdaptor.java:53)
at com.atlassian.mywork.host.service.ClientServiceImpl.getRegistrations(ClientServiceImpl.java:410)
at com.atlassian.mywork.host.service.ClientServiceImpl.updatePotentialClient(ClientServiceImpl.java:373)
at com.atlassian.mywork.host.service.ClientServiceImpl.doInitActiveClients(ClientServiceImpl.java:256)
at com.atlassian.mywork.host.service.ClientServiceImpl.access$600(ClientServiceImpl.java:73)
at com.atlassian.mywork.host.service.ClientServiceImpl$4.doInTransaction(ClientServiceImpl.java:242)
at com.atlassian.mywork.host.service.ClientServiceImpl$4.doInTransaction(ClientServiceImpl.java:239)
at com.atlassian.sal.core.transaction.HostContextTransactionTemplate$1.doInTransaction(HostContextTransactionTemplate.java:21)
at com.atlassian.sal.spring.component.SpringHostContextAccessor$1.doInTransaction(SpringHostContextAccessor.java:71)
at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)
at com.atlassian.sal.spring.component.SpringHostContextAccessor.doInTransaction(SpringHostContextAccessor.java:68)
at com.atlassian.confluence.spring.transaction.interceptor.ConfluenceSpringHostContextAccessor.access$001(ConfluenceSpringHostContextAccessor.java:21)
at com.atlassian.confluence.spring.transaction.interceptor.ConfluenceSpringHostContextAccessor.lambda$doInTransaction$3(ConfluenceSpringHostContextAccessor.java:72)
at com.atlassian.confluence.impl.vcache.VCacheRequestContextManager.doInRequestContextInternal(VCacheRequestContextManager.java:87)
at com.atlassian.confluence.impl.vcache.VCacheRequestContextManager.doInRequestContext(VCacheRequestContextManager.java:71)
at com.atlassian.confluence.spring.transaction.interceptor.ConfluenceSpringHostContextAccessor.doInTransaction(ConfluenceSpringHostContextAccessor.java:72)
at sun.reflect.GeneratedMethodAccessor276.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.atlassian.plugin.util.ContextClassLoaderSettingInvocationHandler.invoke(ContextClassLoaderSettingInvocationHandler.java:26)
at com.sun.proxy.$Proxy320.doInTransaction(Unknown Source)
at sun.reflect.GeneratedMethodAccessor276.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.atlassian.plugin.osgi.bridge.external.HostComponentFactoryBean$DynamicServiceInvocationHandler.invoke(HostComponentFactoryBean.java:136)
at com.sun.proxy.$Proxy320.doInTransaction(Unknown Source)
at com.atlassian.sal.core.transaction.HostContextTransactionTemplate.execute(HostContextTransactionTemplate.java:18)
at sun.reflect.GeneratedMethodAccessor280.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:333)
at org.eclipse.gemini.blueprint.service.importer.support.internal.aop.ServiceInvoker.doInvoke(ServiceInvoker.java:56)
at org.eclipse.gemini.blueprint.service.importer.support.internal.aop.ServiceInvoker.invoke(ServiceInvoker.java:60)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.support.DelegatingIntroductionInterceptor.doProceed(DelegatingIntroductionInterceptor.java:133)
at org.springframework.aop.support.DelegatingIntroductionInterceptor.invoke(DelegatingIntroductionInterceptor.java:121)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.eclipse.gemini.blueprint.service.util.internal.aop.ServiceTCCLInterceptor.invokeUnprivileged(ServiceTCCLInterceptor.java:70)
at org.eclipse.gemini.blueprint.service.util.internal.aop.ServiceTCCLInterceptor.invoke(ServiceTCCLInterceptor.java:53)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.eclipse.gemini.blueprint.service.importer.support.LocalBundleContextAdvice.invoke(LocalBundleContextAdvice.java:57)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.support.DelegatingIntroductionInterceptor.doProceed(DelegatingIntroductionInterceptor.java:133)
at org.springframework.aop.support.DelegatingIntroductionInterceptor.invoke(DelegatingIntroductionInterceptor.java:121)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:213)
at com.sun.proxy.$Proxy1682.execute(Unknown Source)
at com.atlassian.mywork.host.service.ClientServiceImpl.lockAndInitializeActiveClients(ClientServiceImpl.java:239)
at com.atlassian.mywork.host.service.ClientServiceImpl.access$500(ClientServiceImpl.java:73)
at com.atlassian.mywork.host.service.ClientServiceImpl$3.run(ClientServiceImpl.java:225)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

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