org.jvnet.hudson.reactor.ReactorException: hudson.util.IOException2: Unable to read /usr/local/jenkins/config.xml
at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:246)
at jenkins.InitReactorRunner.run(InitReactorRunner.java:43)
at jenkins.model.Jenkins.executeReactor(Jenkins.java:910)
at jenkins.model.Jenkins.<init>(Jenkins.java:806)
at hudson.model.Hudson.<init>(Hudson.java:81)
at hudson.model.Hudson.<init>(Hudson.java:77)
at hudson.WebAppMain$3.run(WebAppMain.java:221)
Caused by: hudson.util.IOException2: Unable to read /usr/local/jenkins/config.xml
at hudson.XmlFile.unmarshal(XmlFile.java:170)
at jenkins.model.Jenkins$17.run(Jenkins.java:2530)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:899)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
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:722)
Caused by: com.thoughtworks.xstream.converters.ConversionException: Could not set field class hudson.security.csrf.DefaultCrumbIssuer.PROXY_HEADER : null : Could not set field class hudson.security.csrf.DefaultCrumbIssuer.PROXY_HEADER : null
---- Debugging information ----
message : Could not set field class hudson.security.csrf.DefaultCrumbIssuer.PROXY_HEADER : null
cause-exception : com.thoughtworks.xstream.converters.reflection.ObjectAccessException
cause-message : Could not set field class hudson.security.csrf.DefaultCrumbIssuer.PROXY_HEADER : null
class : hudson.security.csrf.DefaultCrumbIssuer
required-type : hudson.security.csrf.DefaultCrumbIssuer
converter-type : hudson.util.RobustReflectionConverter
path : /hudson/crumbIssuer/PROXY_HEADER
line number : 219
class[1] : hudson.model.Hudson
version : not available
-------------------------------
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:79)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at hudson.util.RobustReflectionConverter.unmarshalField(RobustReflectionConverter.java:333)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:275)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:222)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:134)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1061)
at hudson.util.XStream2.unmarshal(XStream2.java:109)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1045)
at hudson.XmlFile.unmarshal(XmlFile.java:166)
... 9 more
Caused by: com.thoughtworks.xstream.converters.reflection.ObjectAccessException: Could not set field class hudson.security.csrf.DefaultCrumbIssuer.PROXY_HEADER : null
at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.write(Sun14ReflectionProvider.java:141)
at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.writeField(Sun14ReflectionProvider.java:104)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:290)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:222)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
... 24 more
Caused by: java.lang.IllegalArgumentException
at sun.misc.Unsafe.objectFieldOffset(Native Method)
at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.getFieldOffset(Sun14ReflectionProvider.java:150)
at com.thoughtworks.xstream.converters.reflection.Sun14ReflectionProvider.write(Sun14ReflectionProvider.java:112)
... 28 more

Solution: manually edit config.xml and remove crumb settings will allow jenkins to be accessible again.
The diff shows that the PROXY__HEADER field is not in the config anymore after reconfiguring with proxy support in 1.531