When trying to upgrade the Gerrit Trigger Plugin to its latest version 2.18.2, it fails with the following error message (while other plugins can be upgraded successfully:

SEVERE: Failed to install Gerrit Trigger
hudson.util.IOException2: Failed to download from http://updates.jenkins-ci.org/download/plugins/gerrit-trigger/2.18.2/gerrit-trigger.hpi
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:797)
at hudson.model.UpdateCenter$DownloadJob._run(UpdateCenter.java:1148)
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1309)
at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1126)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:110)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.io.IOException: Server returned HTTP response code: 504 for URL: http://updates.jenkins-ci.org/download/plugins/gerrit-trigger/2.18.2/gerrit-trigger.hpi
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1890)
at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1885)
at java.security.AccessController.doPrivileged(Native Method)
at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1884)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1457)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441)
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:757)
... 7 more
Caused by: java.io.IOException: Server returned HTTP response code: 504 for URL: http://updates.jenkins-ci.org/download/plugins/gerrit-trigger/2.18.2/gerrit-trigger.hpi
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1840)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441)
at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:2943)
at java.net.URLConnection.getHeaderFieldLong(URLConnection.java:629)
at java.net.URLConnection.getContentLengthLong(URLConnection.java:501)
at java.net.URLConnection.getContentLength(URLConnection.java:485)
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:756)
... 7 more