Plugins fail with following error:

hudson.util.IOException2: Failed to download from http://updates.jenkins-ci.org/download/plugins/durable-task/1.3/durable-task.hpi
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:796)
at hudson.model.UpdateCenter$DownloadJob._run(UpdateCenter.java:1143)
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1303)
at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1121)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:104)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.io.IOException: Server returned HTTP response code: 500 for URL: http://updates.jenkins-ci.org/download/plugins/durable-task/1.3/durable-task.hpi
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1514)
at java.security.AccessController.doPrivileged(Native Method)
at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1508)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1162)
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:756)
... 8 more
Caused by: java.io.IOException: Server returned HTTP response code: 500 for URL: http://updates.jenkins-ci.org/download/plugins/durable-task/1.3/durable-task.hpi
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1459)
at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:2340)
at java.net.URLConnection.getHeaderFieldInt(URLConnection.java:579)
at java.net.URLConnection.getContentLength(URLConnection.java:474)
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:755)

This occurs for all plugins.
Jenkins is behind proxy server.
Updates can be downloaded with curl/wget.

Similar to follow issues, which none have had fixes:
https://issues.jenkins-ci.org/browse/JENKINS-9999
https://issues.jenkins-ci.org/browse/JENKINS-10619
https://issues.jenkins-ci.org/browse/JENKINS-7736