Jenkins Won't Start - Unable to read /var/lib/jenkins/config.xml

One afternoon, the error that showed up when I restarted Jenkins

hudson.util.HudsonFailedToLoad: org.jvnet.hudson.reactor.ReactorException: java.io.IOException: Unable to read /var/lib/jenkins/config.xml
	at hudson.WebAppMain$3.run(WebAppMain.java:234)
Caused by: org.jvnet.hudson.reactor.ReactorException: java.io.IOException: Unable to read /var/lib/jenkins/config.xml
	at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:269)
	at jenkins.InitReactorRunner.run(InitReactorRunner.java:44)
	at jenkins.model.Jenkins.executeReactor(Jenkins.java:912)
	at jenkins.model.Jenkins.<init>(Jenkins.java:811)
	at hudson.model.Hudson.<init>(Hudson.java:82)
	at hudson.model.Hudson.<init>(Hudson.java:78)
	at hudson.WebAppMain$3.run(WebAppMain.java:222)

An error saying cannot read /var/lib/jenkins/config.xml.

The owner of /var/lib/jenkins/config.xml is jenkins:jenkins, so why?

After testing and fixing various permission-related things, changing the owner of the entire plugins directory fixed it. Phew.

$ sudo chown -R jenkins:jenkins /var/lib/jenkins/plugins

I never figured out what plugin behavior could have changed the owner.

I found an article about the same issue, so I’m linking it

#764711 - jenkins: Fails on "Unable to read /var/lib/jenkins/config.xml" - Debian Bug report logs