No plugin is displayed in the available tab of the plugin management page. If clicking the Check Now button in the advanced tab, it says: Signature verification failed in update site 'default'

And the following exception is found in the log file:

SEVERE: ERROR: Signature verification failed in update site 'default' <a href='#' class='showDetails'>(show details)</a><pre style='display:none'>java.security.cert.CertPathValidatorException: timestamp check failed
at sun.security.provider.certpath.PKIXMasterCertPathValidator.validate(PKIXMasterCertPathValidator.java:139)
at sun.security.provider.certpath.PKIXCertPathValidator.doValidate(PKIXCertPathValidator.java:330)
at sun.security.provider.certpath.PKIXCertPathValidator.engineValidate(PKIXCertPathValidator.java:178)
at java.security.cert.CertPathValidator.validate(CertPathValidator.java:250)
at org.jvnet.hudson.crypto.CertificateUtil.validatePath(CertificateUtil.java:93)
at jenkins.util.JSONSignatureValidator.verifySignature(JSONSignatureValidator.java:76)
at hudson.model.UpdateSite.verifySignature(UpdateSite.java:219)
at hudson.model.UpdateSite.updateData(UpdateSite.java:198)
at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:170)
at hudson.PluginManager.doCheckUpdatesServer(PluginManager.java:890)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:298)
1006,1 85%
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489)
at org.eclipse.jetty.server.AbstractHttpConnection.content(AbstractHttpConnection.java:960)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.content(AbstractHttpConnection.java:1021)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:865)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:240)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:668)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.security.cert.CertificateExpiredException: NotAfter: Wed Jan 28 05:04:07 CST 2015
at sun.security.x509.CertificateValidity.valid(CertificateValidity.java:256)
at sun.security.x509.X509CertImpl.checkValidity(X509CertImpl.java:570)
at sun.security.provider.certpath.BasicChecker.verifyTimestamp(BasicChecker.java:157)
at sun.security.provider.certpath.BasicChecker.check(BasicChecker.java:109)
at sun.security.provider.certpath.PKIXMasterCertPathValidator.validate(PKIXMasterCertPathValidator.java:117)
... 84 more
</pre>