I see issues with remoting constantly.

I've been gradually upgrading Jenkins - I assume the slave.jar has the new remoting code too, all slaves are ssh-slave to Linux machines.

When I get this the job in question often hangs forever.

<===[JENKINS REMOTING CAPACITY]===>^@@^@channel started
Remoting version: 3.17
This is a Unix agent
Evacuated stdout
Agent successfully connected and online
Mar 14, 2019 8:36:01 AM hudson.Launcher$RemoteLaunchCallable$1 join
INFO: Failed to synchronize IO streams on the channel hudson.remoting.Channel@76ccd017:channel
java.lang.NoClassDefFoundError: hudson/remoting/Channel$IOSyncer
at hudson.remoting.Channel.syncIO(Channel.java:1679)
at hudson.Launcher$RemoteLaunchCallable$1.join(Launcher.java:1315)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:927)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:901)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:850)
at hudson.remoting.UserRequest.perform(UserRequest.java:210)
at hudson.remoting.UserRequest.perform(UserRequest.java:53)
at hudson.remoting.Request$2.run(Request.java:364)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: hudson.remoting.Channel$IOSyncer
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 17 more