This happens after jenkins server is upgraded. Only some git repositories have this issue. jenkins git polling of other repositories on the same server works fine.

Using strategy: Default
[poll] Last Built Revision: Revision ca82a6dff817ec66f44342007202690a93763949 (refs/remotes/origin/master)
> git ls-remote -h ssh://username@host/repository.git master # timeout=10
FATAL: hudson.plugins.git.GitException: Command "git ls-remote -h ssh://username@host/repository.git master" returned status code 128:
stdout:
stderr: error: cannot run ssh: No such file or directory
fatal: unable to fork

hudson.util.IOException2: hudson.plugins.git.GitException: Command "git ls-remote -h ssh://username@host/repository.git master" returned status code 128:
stdout:
stderr: error: cannot run ssh: No such file or directory
fatal: unable to fork

at hudson.plugins.git.GitSCM.compareRemoteRevisionWith(GitSCM.java:462)
at hudson.scm.SCM._compareRemoteRevisionWith(SCM.java:357)
at hudson.scm.SCM.poll(SCM.java:374)
at hudson.model.AbstractProject._poll(AbstractProject.java:1428)
at hudson.model.AbstractProject.poll(AbstractProject.java:1331)
at hudson.triggers.SCMTrigger$Runner.runPolling(SCMTrigger.java:477)
at hudson.triggers.SCMTrigger$Runner.run(SCMTrigger.java:506)
at hudson.util.SequentialExecutionQueue$QueueEntry.run(SequentialExecutionQueue.java:118)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:679)
Caused by: hudson.plugins.git.GitException: Command "git ls-remote -h ssh://username@host/repository.git master" returned status code 128:
stdout:
stderr: error: cannot run ssh: No such file or directory
fatal: unable to fork

at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1407)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:1195)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:1119)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:1110)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.getHeadRev(CliGitAPIImpl.java:2004)
at hudson.plugins.git.GitSCM.compareRemoteRevisionWithImpl(GitSCM.java:495)
at hudson.plugins.git.GitSCM.compareRemoteRevisionWith(GitSCM.java:460)
... 13 more
Done. Took 0.78 sec