Marked all plugins for download and install after restart, then upgraded Jenkins to the latest version.
All pages now show this exception. The Maven plugin wasn't even enabled beforehand.

org.jvnet.hudson.reactor.ReactorException: java.lang.NoClassDefFoundError: hudson/maven/MavenBuild
at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:246)
at jenkins.InitReactorRunner.run(InitReactorRunner.java:43)
at jenkins.model.Jenkins.executeReactor(Jenkins.java:915)
at jenkins.model.Jenkins.<init>(Jenkins.java:811)
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: java.lang.NoClassDefFoundError: hudson/maven/MavenBuild
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Class.java:2475)
at java.lang.Class.getDeclaredMethods(Class.java:1818)
at org.jvnet.hudson.annotation_indexer.Index$2$1.fetch(Index.java:101)
at org.jvnet.hudson.annotation_indexer.Index$2$1.hasNext(Index.java:71)
at org.jvnet.hudson.annotation_indexer.FilterIterator.fetch(FilterIterator.java:23)
at org.jvnet.hudson.annotation_indexer.FilterIterator.hasNext(FilterIterator.java:42)
at hudson.init.TaskMethodFinder.discoverTasks(TaskMethodFinder.java:55)
at hudson.init.InitializerFinder.discoverTasks(InitializerFinder.java:33)
at hudson.PluginManager$2$1$3.run(PluginManager.java:388)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:904)
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$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:680)
Caused by: java.lang.ClassNotFoundException: hudson.maven.MavenBuild
at org.apache.tools.ant.AntClassLoader.findClassInComponents(AntClassLoader.java:1365)
at org.apache.tools.ant.AntClassLoader.findClass(AntClassLoader.java:1315)
at org.apache.tools.ant.AntClassLoader.loadClass(AntClassLoader.java:1068)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
... 18 more