The stacktrace (polluted with timestamps, sorry) says:

13:30:47 java.io.IOException: Unable to serialize RPCRequest(30,withRepository)
13:30:47 at hudson.remoting.UserRequest.serialize(UserRequest.java:166)
13:30:47 at hudson.remoting.UserRequest.<init>(UserRequest.java:62)
13:30:47 at hudson.remoting.Channel.call(Channel.java:712)
13:30:47 at hudson.remoting.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:167)
13:30:47 at com.sun.proxy.$Proxy46.withRepository(Unknown Source)
13:30:47 at org.jenkinsci.plugins.gitclient.RemoteGitImpl.withRepository(RemoteGitImpl.java:217)
13:30:47 at hudson.plugins.git.extensions.impl.JGitCredential.decorate(JGitCredential.java:67)
13:30:47 at hudson.plugins.git.GitSCM.createClient(GitSCM.java:560)
13:30:47 at hudson.plugins.git.GitSCM.createClient(GitSCM.java:550)
13:30:47 at hudson.plugins.git.GitSCM.checkout(GitSCM.java:825)
13:30:47 at hudson.model.AbstractProject.checkout(AbstractProject.java:1394)
13:30:47 at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:676)
13:30:47 at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
13:30:47 at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:581)
13:30:47 at hudson.model.Run.execute(Run.java:1593)
13:30:47 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
13:30:47 at hudson.model.ResourceController.execute(ResourceController.java:88)
13:30:47 at hudson.model.Executor.run(Executor.java:247)
13:30:47 Caused by: java.io.NotSerializableException: hudson.plugins.git.extensions.impl.JGitCredential
13:30:47 at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1180)
13:30:47 at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1528)
13:30:47 at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1493)
13:30:47 at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1416)
13:30:47 at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
13:30:47 at java.io.ObjectOutputStream.writeArray(ObjectOutputStream.java:1362)
13:30:47 at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1170)
13:30:47 at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1528)
13:30:47 at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1493)
13:30:47 at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1416)
13:30:47 at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
13:30:47 at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
13:30:47 at hudson.remoting.UserRequest._serialize(UserRequest.java:155)
13:30:47 at hudson.remoting.UserRequest.serialize(UserRequest.java:164)
13:30:47 ... 17 more
13:30:47 Finished: FAILURE