Nicely summed up here:

http://jenkins.361315.n4.nabble.com/subversion-plugin-not-for-concurrent-builds-td4636014.html

A race condition or two exist where two concurrent builds both try to delete svnexternals.txt from the project root. Surely this file belongs in the workspace for the particular build?

When this happens you get:

java.io.IOException: Unable to delete /var/lib/jenkins/jobs/<job>/svnexternals.txt
at hudson.util.AtomicFileWriter.commit(AtomicFileWriter.java:112)
at hudson.XmlFile.write(XmlFile.java:178)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:700)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1256)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:590)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:495)
at hudson.model.Run.execute(Run.java:1502)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:237)