System is configured to use an SSH agent to get the build from the Git server, and build on the build agent. The system works intermittently, and when it doesn't, the error reported is:
Started by an SCM change
Started by an SCM change
Started by an SCM change
Building remotely on Build Agent 1 in workspace C:\Jenkins\workspace\Fusion 3.69
FATAL: java.io.IOException: Remote call on Build Agent 1 failed
hudson.remoting.RemotingSystemException: java.io.IOException: Remote call on Build Agent 1 failed
at hudson.remoting.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:183)
at com.sun.proxy.$Proxy75.addCredentials(Unknown Source)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl.addCredentials(RemoteGitImpl.java:196)
at hudson.plugins.git.GitSCM.createClient(GitSCM.java:584)
at hudson.plugins.git.GitSCM.createClient(GitSCM.java:561)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:866)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1411)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:652)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:557)
at hudson.model.Run.execute(Run.java:1665)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:230)
Caused by: java.io.IOException: Remote call on Build Agent 1 failed
at hudson.remoting.Channel.call(Channel.java:723)
at hudson.remoting.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:167)
... 13 more
Caused by: java.lang.Error: Failed to deserialize the Callable object.
at hudson.remoting.UserRequest.perform(UserRequest.java:104)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at hudson.remoting.Engine$1$1.run(Engine.java:58)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCastException: cannot assign instance of com.cloudbees.plugins.credentials.CredentialsScope$2 to field com.cloudbees.plugins.credentials.BaseCredentials.scope of type com.cloudbees.plugins.credentials.CredentialsScope in instance of com.cloudbees.jenkins.plugins.sshcredentials.impl.BasicSSHUserPrivateKey
at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(Unknown Source)
at java.io.ObjectStreamClass.setObjFieldValues(Unknown Source)
at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readArray(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
at java.io.ObjectInputStream.readSerialData(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at hudson.remoting.UserRequest.deserialize(UserRequest.java:182)
at hudson.remoting.UserRequest.perform(UserRequest.java:98)
... 9 more

This seems similar to a few other SSH/Git related issues that are there, so any patches and/or pointers to fix this would be appreciated.