After update to 1.6.1 I now get the following:

Building on master in workspace /usr/local/NAS/jenkins_jobs/master-Documentation/workspace

Deleting project workspace... done

FATAL: org.jenkinsci.plugins.gitclient.GitClient
java.lang.NoClassDefFoundError: org.jenkinsci.plugins.gitclient.GitClient
at org.jenkinsci.plugins.gitclient.Git$1.invoke(Git.java:63)
at org.jenkinsci.plugins.gitclient.Git$1.invoke(Git.java:54)
at hudson.FilePath.act(FilePath.java:914)
at hudson.FilePath.act(FilePath.java:887)
at org.jenkinsci.plugins.gitclient.Git.getClient(Git.java:66)
at hudson.plugins.git.GitSCM.createClient(GitSCM.java:569)
at hudson.plugins.git.GitSCM.createClient(GitSCM.java:561)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:866)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1411)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:651)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:560)
at hudson.model.Run.execute(Run.java:1670)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)

Reverting to 1.6.0 resolves the issue.