hudson.plugins.statusmonitor.MonitorAction imports javax.servlet.jsp.jstl.core.LoopTagStatus, which is no longer available now that Jenkins has moved to EE 9. As a result the /monitor page does not load, and since MonitorAction is a RootAction a link to it can be rendered in the sidebar on some pages and thus breaks those pages too (all of which show up as an "Oops!", either for the whole page or nested in the sidebar). For some reason restarting our Jenkins instance works for a while at first though.