How to configure Jenkins with Nginx to avoid "Cannot allocate memory" issue

I have Jenkins running behind a Nginx proxy on my Ubuntu 16.04 server. I have connected it with GitHub through webhooks, and the connection works perfectly, however every build fails, and I keep getting the same error log:

Started by user admin
Building in workspace /var/lib/jenkins/workspace/kudos.gaboratorium.com
[WS-CLEANUP] Deleting project workspace...
[WS-CLEANUP] Done
Cloning the remote Git repository
Cloning repository https://github.com/gaboratorium/kudos.git
 > /usr/bin/git init /var/lib/jenkins/workspace/kudos.gaboratorium.com # timeout=10
Fetching upstream changes from https://github.com/gaboratorium/kudos.git
 > /usr/bin/git --version # timeout=10
 > /usr/bin/git fetch --tags --progress https://github.com/gaboratorium/kudos.git +refs/heads/*:refs/remotes/origin/* # timeout=15
ERROR: Error cloning remote repo 'origin'
hudson.plugins.git.GitException: Command "/usr/bin/git fetch --tags --progress https://github.com/gaboratorium/kudos.git +refs/heads/*:refs/remotes/origin/*" returned status code 128:
stdout: 
stderr: error: cannot fork() for fetch-pack: Cannot allocate memory

	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1903)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:1622)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$300(CliGitAPIImpl.java:71)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$1.execute(CliGitAPIImpl.java:348)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$2.execute(CliGitAPIImpl.java:545)
	at hudson.plugins.git.GitSCM.retrieveChanges(GitSCM.java:1070)
	at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1110)
	at hudson.scm.SCM.checkout(SCM.java:495)
	at hudson.model.AbstractProject.checkout(AbstractProject.java:1276)
	at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:560)
	at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:485)
	at hudson.model.Run.execute(Run.java:1735)
	at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
	at hudson.model.ResourceController.execute(ResourceController.java:97)
	at hudson.model.Executor.run(Executor.java:405)
ERROR: Error cloning remote repo 'origin'
Finished: FAILURE

I can’t figure out what is going on, because everything seems to work fine. The strange thing is that if I log in to my server as jenkins and I perform these steps manually, everything succeeds.

In addition, Jenkins breaks occasionally and from that point Nginx keeps giving me a 503 error. If I restart the Jenkins service, everything works again.