Here is the error shown in the logs (not that it also occur from time to time when I manually launch the build.).

Jan 11, 2016 1:28:01 PM SEVERE hudson.triggers.SCMTrigger$Runner runPolling
Failed to record SCM polling for hudson.model.FreeStyleProject@3ad7e6a5[BuildIPadForTestFairy]
java.lang.NoClassDefFoundError: hudson/tasks/Mailer$UserProperty
at hudson.plugins.tfs.model.Project.getOrCreateUserLookup(Project.java:112)
at hudson.plugins.tfs.model.Project.getVCCHistory(Project.java:81)
at hudson.plugins.tfs.model.Project.getLatestChangeset(Project.java:165)
at hudson.plugins.tfs.TeamFoundationServerScm.compareRemoteRevisionWith(TeamFoundationServerScm.java:498)
at hudson.scm.SCM.poll(SCM.java:398)
at hudson.model.AbstractProject._poll(AbstractProject.java:1453)
at hudson.model.AbstractProject.poll(AbstractProject.java:1356)
at hudson.triggers.SCMTrigger$Runner.runPolling(SCMTrigger.java:526)
at hudson.triggers.SCMTrigger$Runner.run(SCMTrigger.java:555)
at hudson.util.SequentialExecutionQueue$QueueEntry.run(SequentialExecutionQueue.java:119)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: hudson.tasks.Mailer$UserProperty
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)
... 15 more