Clone works with 3.9.1. I restored this version.

Checking out Revision 3798afd32ccac880988e2c065f0445454e42dd95 (origin/master)
Commit message: "#DC Added commits from SIG-Signa SIG-BIN-TestDaten to master"
FATAL: not implemented yet
Also: hudson.remoting.Channel$CallSiteStackTrace: Remote call to JNLP4-connect connection from kie-jenkinsbld9.ceu.corp.heidelberg.com/10.51.200.115:57675
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1741)
at hudson.remoting.UserResponse.retrieve(UserRequest.java:389)
at hudson.remoting.Channel.call(Channel.java:955)
at hudson.remoting.RemoteInvocationHandler.invoke(RemoteInvocationHandler.java:283)
at com.sun.proxy.$Proxy114.setupSubmoduleUrls(Unknown Source)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl.setupSubmoduleUrls(RemoteGitImpl.java:655)
at hudson.plugins.git.extensions.impl.SubmoduleOption.onCheckoutCompleted(SubmoduleOption.java:101)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1257)
at hudson.scm.SCM.checkout(SCM.java:504)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1208)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:574)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
at hudson.model.Run.execute(Run.java:1819)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:429)
java.lang.UnsupportedOperationException: not implemented yet
at org.jenkinsci.plugins.gitclient.JGitAPIImpl.setupSubmoduleUrls(JGitAPIImpl.java:2413)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:922)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:896)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:853)
at hudson.remoting.UserRequest.perform(UserRequest.java:207)
at hudson.remoting.UserRequest.perform(UserRequest.java:53)
at hudson.remoting.Request$2.run(Request.java:358)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at hudson.remoting.Engine$1$1.run(Engine.java:98)
at java.lang.Thread.run(Unknown Source)