Nexus Crowd plugin stopped working after 1.9.0.1 upgrade

After upgrading Nexus OSS 1.7.1 to 1.9.0.1, Crowd plugin stopped letting Crowd authenticated users log into the system. Others have also reported similar problems at https://docs.sonatype.org/display/Nexus/Nexus+Crowd+Plugin page. Upgrading Crowd plugin to 1.6.1 did not help. Nexus OSS 1.9.0.1 with Crowd plugin 1.6.1 outputs the following exception during start-up:

2011-03-02 12:12:40 ERROR [tp-19257644-149] - o.s.s.DefaultSecuri~ - Unable to lookup security realms
java.lang.ClassNotFoundException: Crowd
at org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy.loadClass(SelfFirstStrategy.java:50)
at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:244)
at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:230)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:169)
at org.sonatype.security.DefaultSecuritySystem.getRealmsFromConfigSource(DefaultSecuritySystem.java:223)
at org.sonatype.security.DefaultSecuritySystem.setSecurityManagerRealms(DefaultSecuritySystem.java:878)
at org.sonatype.security.DefaultSecuritySystem.setRealms(DefaultSecuritySystem.java:816)
at org.sonatype.nexus.configuration.application.DefaultNexusConfiguration.setRealms(DefaultNexusConfiguration.java:417)
at org.sonatype.nexus.rest.global.GlobalConfigurationPlexusResource.put(GlobalConfigurationPlexusResource.java:378)
at org.sonatype.plexus.rest.resource.RestletResource.storeRepresentation(RestletResource.java:316)
at org.sonatype.nexus.rest.NexusRestletResource.storeRepresentation(NexusRestletResource.java:102)
at org.restlet.resource.Resource.put(Resource.java:706)
at org.restlet.resource.Resource.handlePut(Resource.java:603)
at org.restlet.Finder.handle(Finder.java:359)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Router.handle(Router.java:504)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Router.handle(Router.java:504)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.sonatype.plexus.rest.RetargetableRestlet.doHandle(RetargetableRestlet.java:39)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Filter.doHandle(Filter.java:150)
at com.noelios.restlet.StatusFilter.doHandle(StatusFilter.java:130)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at com.noelios.restlet.ChainHelper.handle(ChainHelper.java:124)
at com.noelios.restlet.application.ApplicationHelper.handle(ApplicationHelper.java:112)
at org.restlet.Application.handle(Application.java:341)
at org.restlet.ext.wadl.WadlApplication.handle(WadlApplication.java:705)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Router.handle(Router.java:504)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at org.restlet.Router.handle(Router.java:504)
at org.restlet.Filter.doHandle(Filter.java:150)
at org.restlet.Filter.handle(Filter.java:195)
at com.noelios.restlet.ChainHelper.handle(ChainHelper.java:124)
at org.restlet.Component.handle(Component.java:673)
at org.restlet.Server.handle(Server.java:331)
at com.noelios.restlet.ServerHelper.handle(ServerHelper.java:68)
at com.noelios.restlet.http.HttpServerHelper.handle(HttpServerHelper.java:147)
at com.noelios.restlet.ext.servlet.ServerServlet.service(ServerServlet.java:881)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:502)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:62)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:67)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:67)
at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:109)
at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:138)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:83)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:67)
at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:109)
at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:138)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:83)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:67)
at org.apache.shiro.web.servlet.AbstractShiroFilter.executeChain(AbstractShiroFilter.java:359)
at org.apache.shiro.web.servlet.AbstractShiroFilter$1.call(AbstractShiroFilter.java:275)
at org.apache.shiro.subject.support.SubjectCallable.doCall(SubjectCallable.java:90)
at org.apache.shiro.subject.support.SubjectCallable.call(SubjectCallable.java:83)
at org.apache.shiro.subject.support.DelegatingSubject.execute(DelegatingSubject.java:343)
at org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal(AbstractShiroFilter.java:272)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:83)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1148)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:387)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:417)
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:534)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:879)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:747)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:520)