I upgraded from 2.46.3 to 2.60.1 like so:

sdonner@ip-172-31-58-216:~$ sudo apt-get --only-upgrade install jenkins
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  gconf-service gconf-service-backend gconf2 gconf2-common libavahi-glib1
  libbonobo2-0 libbonobo2-common libcanberra0 libgconf-2-4 libgconf2-4
  libgnome2-0 libgnome2-bin libgnome2-common libgnomevfs2-0
  libgnomevfs2-common libidl-common libidl0 liborbit-2-0 liborbit2 libtdb1
  libvorbisfile3 sound-theme-freedesktop
Use 'apt-get autoremove' to remove them.
The following packages will be upgraded:
  jenkins
1 upgraded, 0 newly installed, 0 to remove and 25 not upgraded.
Need to get 70.3 MB of archives.
After this operation, 270 kB of additional disk space will be used.
Get:1 http://pkg.jenkins.io/debian-stable/ binary/ jenkins 2.60.1 [70.3 MB]
Fetched 70.3 MB in 3s (20.5 MB/s)   
(Reading database ... 71808 files and directories currently installed.)
Preparing to unpack .../jenkins_2.60.1_all.deb ...
 * Stopping Jenkins Automation Server jenkins                            [ OK ]
Unpacking jenkins (2.60.1) over (2.46.3) ...
Processing triggers for ureadahead (0.100.0-16) ...
Setting up jenkins (2.60.1) ...
 * Starting Jenkins Automation Server jenkins                            [ OK ] 

And now we're seeing this exception, and Jenkins is throwing a 503:

Jun 21, 2017 9:58:13 AM org.jenkinsci.plugins.workflow.cps.CpsFlowExecution getCurrentHeads
WARNING: null
java.lang.IllegalStateException: List of flow heads unset for CpsFlowExecution[null]
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.getCurrentHeads(CpsFlowExecution.java:794)
at org.jenkinsci.plugins.workflow.flow.FlowExecution.isComplete(FlowExecution.java:176)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.isComplete(CpsFlowExecution.java:1016)
at org.jenkinsci.plugins.pipeline.modeldefinition.SyntheticStageGraphListener$GraphListenerAction.attachListener(SyntheticStageGraphListener.java:102)
at org.jenkinsci.plugins.pipeline.modeldefinition.SyntheticStageGraphListener$GraphListenerAction.onLoad(SyntheticStageGraphListener.java:87)
at hudson.model.Run.onLoad(Run.java:356)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.onLoad(WorkflowRun.java:576)
at hudson.model.RunMap.retrieve(RunMap.java:224)
at hudson.model.RunMap.retrieve(RunMap.java:56)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap.java:500)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap.java:482)
at jenkins.model.lazy.AbstractLazyLoadRunMap.getByNumber(AbstractLazyLoadRunMap.java:380)
at jenkins.model.lazy.AbstractLazyLoadRunMap.search(AbstractLazyLoadRunMap.java:345)
at jenkins.model.lazy.LazyBuildMixIn$RunMixIn.getPreviousBuild(LazyBuildMixIn.java:362)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.getPreviousBuild(WorkflowRun.java:206)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.getPreviousBuild(WorkflowRun.java:125)
at hudson.model.RunMap$1.next(RunMap.java:109)
at hudson.model.RunMap$1.next(RunMap.java:98)
at com.google.common.collect.Iterators$5.next(Iterators.java:554)
at jenkins.widgets.HistoryPageFilter.addInternal(HistoryPageFilter.java:157)
at jenkins.widgets.HistoryPageFilter.add(HistoryPageFilter.java:135)
at hudson.widgets.BuildHistoryWidget.getHistoryPageFilter(BuildHistoryWidget.java:77)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.commons.jexl.util.PropertyExecutor.execute(PropertyExecutor.java:125)
at org.apache.commons.jexl.util.introspection.UberspectImpl$VelGetterImpl.invoke(UberspectImpl.java:314)
at org.apache.commons.jexl.parser.ASTArrayAccess.evaluateExpr(ASTArrayAccess.java:185)
at org.apache.commons.jexl.parser.ASTIdentifier.execute(ASTIdentifier.java:75)
at org.apache.commons.jexl.parser.ASTReference.execute(ASTReference.java:83)
at org.apache.commons.jexl.parser.ASTReference.value(ASTReference.java:57)
at org.apache.commons.jexl.parser.ASTReferenceExpression.value(ASTReferenceExpression.java:51)
at org.apache.commons.jexl.ExpressionImpl.evaluate(ExpressionImpl.java:80)
at hudson.ExpressionFactory2$JexlExpression.evaluate(ExpressionFactory2.java:74)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$3.run(CoreTagLibrary.java:134)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105)
at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95)
at org.kohsuke.stapler.jelly.IncludeTag.doTag(IncludeTag.java:147)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161)
at org.apache.commons.jelly.tags.core.ForEachTag.doTag(ForEachTag.java:150)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99)
at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$1.run(CoreTagLibrary.java:98)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105)
at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105)
at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95)
at org.kohsuke.stapler.jelly.IncludeTag.doTag(IncludeTag.java:147)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99)
at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$1.run(CoreTagLibrary.java:98)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105)
at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105)
at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95)
at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:63)
at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:53)
at org.kohsuke.stapler.jelly.JellyClassTearOff.serveIndexJelly(JellyClassTearOff.java:112)
at org.kohsuke.stapler.jelly.JellyFacet.handleIndexRequest(JellyFacet.java:140)
at org.kohsuke.stapler.IndexViewDispatcher.dispatch(IndexViewDispatcher.java:30)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845)
at org.kohsuke.stapler.MetaClass$5.doDispatch(MetaClass.java:248)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:135)
at org.jenkinsci.plugins.ssegateway.Endpoint$SSEListenChannelFilter.doFilter(Endpoint.java:225)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at com.cloudbees.jenkins.support.slowrequest.SlowRequestFilter.doFilter(SlowRequestFilter.java:37)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at com.smartcodeltd.jenkinsci.plugin.assetbundler.filters.LessCSS.doFilter(LessCSS.java:47)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at io.jenkins.blueocean.ResourceCacheControl.doFilter(ResourceCacheControl.java:134)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:59)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at jenkins.metrics.impl.MetricsFilter.doFilter(MetricsFilter.java:125)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:126)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:553)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
at org.eclipse.jetty.server.Server.handle(Server.java:499)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

Wonder if this is related to/another instance of https://issues.jenkins-ci.org/browse/JENKINS-43055?