Jenkins Not to Starting After Update

I have a Jenkins instance (running on Ubuntu 16.04.3) that I've kept more or less up to date over the last couple of years. Today I tried updating via aptitude (apt-get dist-upgrade) only to get this exception:

com.thoughtworks.xstream.mapper.CannotResolveClassException: org.jenkinsci.plugins.GithubSecurityRealm
    at com.thoughtworks.xstream.mapper.DefaultMapper.realClass(DefaultMapper.java:79)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.DynamicProxyMapper.realClass(DynamicProxyMapper.java:55)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.PackageAliasingMapper.realClass(PackageAliasingMapper.java:88)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.ClassAliasingMapper.realClass(ClassAliasingMapper.java:79)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.ArrayMapper.realClass(ArrayMapper.java:74)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.SecurityMapper.realClass(SecurityMapper.java:71)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at hudson.util.XStream2$CompatibilityMapper.realClass(XStream2.java:285)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at hudson.util.xstream.MapperDelegate.realClass(MapperDelegate.java:43)
    at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:30)
    at com.thoughtworks.xstream.mapper.CachingMapper.realClass(CachingMapper.java:48)
    at hudson.util.RobustReflectionConverter.determineType(RobustReflectionConverter.java:461)
    at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:327)
Caused: jenkins.util.xstream.CriticalXStreamException: org.jenkinsci.plugins.GithubSecurityRealm : org.jenkinsci.plugins.GithubSecurityRealm
---- Debugging information ----
message             : org.jenkinsci.plugins.GithubSecurityRealm
cause-exception     : com.thoughtworks.xstream.mapper.CannotResolveClassException
cause-message       : org.jenkinsci.plugins.GithubSecurityRealm
class               : hudson.model.Hudson
required-type       : hudson.model.Hudson
converter-type      : hudson.util.RobustReflectionConverter
path                : /hudson/securityRealm
line number         : 85
version             : not available
-------------------------------
    at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:356)
    at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:270)
    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:1189)
    at hudson.util.XStream2.unmarshal(XStream2.java:114)
    at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1173)
    at hudson.XmlFile.unmarshal(XmlFile.java:165)
Caused: java.io.IOException: Unable to read /mnt/tank/jenkins/home/config.xml
    at hudson.XmlFile.unmarshal(XmlFile.java:167)
    at jenkins.model.Jenkins.loadConfig(Jenkins.java:3031)
    at jenkins.model.Jenkins.access$1200(Jenkins.java:304)
    at jenkins.model.Jenkins$14.run(Jenkins.java:3049)
    at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
    at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
    at jenkins.model.Jenkins$5.runTask(Jenkins.java:1068)
    at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
    at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
Caused: org.jvnet.hudson.reactor.ReactorException
    at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:269)
    at jenkins.InitReactorRunner.run(InitReactorRunner.java:48)
    at jenkins.model.Jenkins.executeReactor(Jenkins.java:1102)
    at jenkins.model.Jenkins.<init>(Jenkins.java:904)
    at hudson.model.Hudson.<init>(Hudson.java:86)
    at hudson.model.Hudson.<init>(Hudson.java:82)
    at hudson.WebAppMain$3.run(WebAppMain.java:233)
Caused: hudson.util.HudsonFailedToLoad
    at hudson.WebAppMain$3.run(WebAppMain.java:250)

I'm now on 2.96. I'm not sure what version I was on before.

I saw this post regarding Jenkins restart error but in my case this file doesn't exist (find ~/ -name github-oauth.jpi.disabled comes up empty).

I tried for p in $HOME/plugins/*.jpi; do touch $p.disabled; done. When I do that, the server complains about missing the matrix auth plugin. Re-enabling that single plugin causes the error to display again.