Full error is shown below. I believe the scenario was as follows:

Master on Linux, agent on  Windows server, agent launched via DCOM, running service as local user, keep this agent on-line as much as possible.

Linux server was restarted (init 6) while Jenkins was running, no Jenkins shutdown.

Windows node and Jenkins agent service remained running. After restart, Jenkins auto starts, but cannot restart agent.

I suppose it should detect the agent is still running, then restart it instead of start ?

Stopping the  agent service on the windows server and launching via Jenkins UI brought it back on-line.

ERROR: Unexpected error in launching an agent. This is probably a bug in Jenkins
org.jinterop.dcom.common.JIException: Service Already Running
 at org.jvnet.hudson.wmi.Win32Service$Implementation.start(Win32Service.java:149)
Caused: java.lang.reflect.InvocationTargetException
 at sun.reflect.GeneratedMethodAccessor221.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:498)
 at org.kohsuke.jinterop.JInteropInvocationHandler.invoke(JInteropInvocationHandler.java:140)
Caused: java.lang.reflect.UndeclaredThrowableException
 at com.sun.proxy.$Proxy73.start(Unknown Source)
 at hudson.os.windows.ManagedWindowsServiceLauncher.launch(ManagedWindowsServiceLauncher.java:342)
 at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:294)
 at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
 at jenkins.security.ImpersonatingExecutorService$2.call(ImpersonatingExecutorService.java:71)
 at java.util.concurrent.FutureTask.run(FutureTask.java:266)
 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
 at java.lang.Thread.run(Thread.java:748)