My slave is unable to clone from Git over HTTPS (via Git plugin version 2.2.1, Git client plugin version 1.9.0), due to the server's self signed certificate. The reason is that Java doesn't find the server's certificate in its store, even though the Git command line client has no problems with it. The error looks as follows:

FATAL: Failed to fetch from https://myserver/repo.git
hudson.plugins.git.GitException: Failed to fetch from https://myserver/repo.git
at hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:623)
at hudson.plugins.git.GitSCM.retrieveChanges(GitSCM.java:855)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:880)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1251)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:604)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:513)
at hudson.model.Run.execute(Run.java:1706)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)
Caused by: hudson.plugins.git.GitException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.checkCredentials(CliGitAPIImpl.java:1964)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:1143)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$200(CliGitAPIImpl.java:87)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$1.execute(CliGitAPIImpl.java:257)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$1.call(RemoteGitImpl.java:153)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$1.call(RemoteGitImpl.java:146)
at hudson.remoting.UserRequest.perform(UserRequest.java:118)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:328)
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:63)
at java.lang.Thread.run(Unknown Source)

See also StackOverflow: http://stackoverflow.com/questions/23678127/how-do-i-connect-to-a-jenkins-slave-to-a-git-server-with-self-signed-certificate.