I upgraded to 1.13. My builds now get the following error:
java.io.IOException: Remote call on Docker Slave-e37abcfefda7 failed
at hudson.remoting.Channel.call(Channel.java:789)
at hudson.remoting.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:249)
at com.sun.proxy.$Proxy100.addIdentity(Unknown Source)
at com.cloudbees.jenkins.plugins.sshagent.SSHAgentBuildWrapper$SSHAgentEnvironment.add(SSHAgentBuildWrapper.java:395)
at com.cloudbees.jenkins.plugins.sshagent.SSHAgentBuildWrapper.preCheckout(SSHAgentBuildWrapper.java:229)
at jenkins.scm.SCMCheckoutStrategy.preCheckout(SCMCheckoutStrategy.java:76)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:528)
at hudson.model.Run.execute(Run.java:1741)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)
Caused by: java.lang.NoClassDefFoundError: org/bouncycastle/openssl/PEMReader
at jenkins.bouncycastle.api.PEMEncodable.decode(PEMEncodable.java:146)
at com.cloudbees.jenkins.plugins.sshagent.jna.JNRRemoteAgent.addIdentity(JNRRemoteAgent.java:79)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:884)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:859)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:818)
at hudson.remoting.UserRequest.perform(UserRequest.java:152)
at hudson.remoting.UserRequest.perform(UserRequest.java:50)
at hudson.remoting.Request$2.run(Request.java:332)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
at ......remote call to Docker Slave-e37abcfefda7(Native Method)
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1416)
at hudson.remoting.UserResponse.retrieve(UserRequest.java:252)
at hudson.remoting.Channel.call(Channel.java:781)
... 10 more
Caused by: java.lang.ClassNotFoundException: org.bouncycastle.openssl.PEMReader
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1376)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1326)
at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1079)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at hudson.remoting.RemoteClassLoader$ClassLoaderProxy.fetch4(RemoteClassLoader.java:761)
at hudson.remoting.RemoteClassLoader$ClassLoaderProxy.fetch3(RemoteClassLoader.java:803)
at sun.reflect.GeneratedMethodAccessor132.invoke(Unknown Source)
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:884)
at hudson.remoting.Request$2.run(Request.java:332)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at org.jenkinsci.remoting.CallableDecorator.call(CallableDecorator.java:18)
at hudson.remoting.CallableDecoratorList$1.call(CallableDecoratorList.java:21)
at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
at ......remote call to channel(Native Method)
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1416)
at hudson.remoting.Request.call(Request.java:172)
at hudson.remoting.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:252)
at com.sun.proxy.$Proxy5.fetch3(Unknown Source)
at hudson.remoting.RemoteClassLoader.findClass(RemoteClassLoader.java:171)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at jenkins.bouncycastle.api.PEMEncodable.decode(PEMEncodable.java:146)
at com.cloudbees.jenkins.plugins.sshagent.jna.JNRRemoteAgent.addIdentity(JNRRemoteAgent.java:79)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:884)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:859)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:818)
at hudson.remoting.UserRequest.perform(UserRequest.java:152)
at hudson.remoting.UserRequest.perform(UserRequest.java:50)
at hudson.remoting.Request$2.run(Request.java:332)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

Downgrading to 1.10 is a temporary workaround.