My situation is 6.5 upgrade to 6.7u3

Symptoms

Accessing the vSphere Client (HTML5) of vCenter server in browser is failed with error message:

503 Service Unavailable (Failed to connect to endpoint: [class Vmacore::Http::LocalServiceSpec:0x0000025c4df77820] _serverNamespace = /ui action = Allow _port = 5091)

Services status in vCenter server with command "service-control --status --all" shows the "vsphere-ui" service in STOPPED status.

the vsphere_client_virgo.log file, you see entries similar to:

[2020-01-30T01:30:37.230+08:00] [INFO ] cm-catalog-manager-pool-46    com.vmware.vise.vim.lookup.LsCatalogManager                       Time to load properties (ms)C:\ProgramData\VMware\vCenterServer\cfg\vsphere-ui\cmCatalog\com.vmware.cis.cs.eam.zip:US: 2

[2020-01-30T01:30:37.931+08:00] [ERROR] SimpleAsyncTaskExecutor-1     com.vmware.vise.extensionfw.ExtensionManager                      Error retrieving deployment data for the plugin package at C:\ProgramData\VMware\vCenterServer\runtime\vsphere-ui\plugin-packages\vsphere-client

[2020-01-30T01:30:37.942+08:00] [ERROR] SimpleAsyncTaskExecutor-1     com.vmware.vise.util.concurrent.WorkerThread                      SimpleAsyncTaskExecutor-1 terminated with exception:  java.lang.NoClassDefFoundError: java/util/concurrent/ExecutorCompletionService

at com.vmware.vise.extensionfw.impl.PackagesDeployer.<init>(PackagesDeployer.java:160)

at com.vmware.vise.extensionfw.impl.PackagesDeployer.deploy(PackagesDeployer.java:143)

at com.vmware.vise.extensionfw.ExtensionManager.addPluginPackages(ExtensionManager.java:520)

at com.vmware.vise.extensionfw.impl.ExtensionServiceImpl.addPluginPackages(ExtensionServiceImpl.java:164)

at com.vmware.vise.extensionfw.impl.CorePluginsDeploymentActivator.deployPluginsFromDir(CorePluginsDeploymentActivator.java:156)

at com.vmware.vise.extensionfw.impl.CorePluginsDeploymentActivator.deployInternal(CorePluginsDeploymentActivator.java:102)

at com.vmware.vise.extensionfw.impl.CorePluginsDeploymentActivator.access$000(CorePluginsDeploymentActivator.java:27)

at com.vmware.vise.extensionfw.impl.CorePluginsDeploymentActivator$1.run(CorePluginsDeploymentActivator.java:61)

at java.lang.Thread.run(Thread.java:748)

Caused by: java.lang.ClassNotFoundException: Illegal access: this web application instance has been stopped already. Could not load [java.util.concurrent.ExecutorCompletionService]. The following stack trace is thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access.

at org.apache.catalina.loader.WebappClassLoaderBase.checkStateForClassLoading(WebappClassLoaderBase.java:1370)

at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1220)

at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1182)

at org.eclipse.equinox.servletbridge.FrameworkLauncher$ChildFirstURLClassLoader.loadClass0(FrameworkLauncher.java:1051)

at org.eclipse.equinox.servletbridge.FrameworkLauncher$ChildFirstURLClassLoader.loadClass(FrameworkLauncher.java:1036)

at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:371)

at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:364)

at org.eclipse.osgi.internal.loader.ModuleClassLoader.loadClass(ModuleClassLoader.java:161)

at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

... 9 common frames omitted

Caused by: java.lang.IllegalStateException: Illegal access: this web application instance has been stopped already. Could not load [java.util.concurrent.ExecutorCompletionService]. The following stack trace is thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access.

at org.apache.catalina.loader.WebappClassLoaderBase.checkStateForResourceLoading(WebappClassLoaderBase.java:1380)

at org.apache.catalina.loader.WebappClassLoaderBase.checkStateForClassLoading(WebappClassLoaderBase.java:1368)

... 18 common frames omitted