Installed Jenkins using the Mac OS X installer; got it running OK, then I mistakenly clicked on "Enable auto refresh" while plugins were installing, and am now getting:
org.jvnet.hudson.reactor.ReactorException: com.thoughtworks.xstream.mapper.CannotResolveClassException: html : html
at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:246)
at jenkins.model.Jenkins.executeReactor(Jenkins.java:816)
at jenkins.model.Jenkins.<init>(Jenkins.java:730)
at hudson.model.Hudson.<init>(Hudson.java:81)
at hudson.model.Hudson.<init>(Hudson.java:77)
at hudson.WebAppMain$2.run(WebAppMain.java:217)
Caused by: com.thoughtworks.xstream.mapper.CannotResolveClassException: html : html
at com.thoughtworks.xstream.mapper.DefaultMapper.realClass(DefaultMapper.java:68)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.DynamicProxyMapper.realClass(DynamicProxyMapper.java:71)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.PackageAliasingMapper.realClass(PackageAliasingMapper.java:88)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.ClassAliasingMapper.realClass(ClassAliasingMapper.java:86)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.ArrayMapper.realClass(ArrayMapper.java:96)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at hudson.util.XStream2$CompatibilityMapper.realClass(XStream2.java:211)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at hudson.util.xstream.MapperDelegate.realClass(MapperDelegate.java:43)
at com.thoughtworks.xstream.mapper.MapperWrapper.realClass(MapperWrapper.java:38)
at com.thoughtworks.xstream.mapper.CachingMapper.realClass(CachingMapper.java:56)
at com.thoughtworks.xstream.core.util.HierarchicalStreams.readClassType(HierarchicalStreams.java:29)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:136)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:33)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:926)
at hudson.util.XStream2.unmarshal(XStream2.java:91)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:912)
at hudson.XmlFile.unmarshal(XmlFile.java:156)
at jenkins.model.Jenkins$13.run(Jenkins.java:2343)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$5.runTask(Jenkins.java:798)
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$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:680)