Since upgrading to 1.576 (from 1.561 IIRC), I'm getting NPE on different pages. When I trigger a specific job, I'm getting the following stacktrace :
FATAL: null
java.lang.NullPointerException
at java.lang.String$CaseInsensitiveComparator.compare(String.java:1177)
at java.lang.String$CaseInsensitiveComparator.compare(String.java:1170)
at java.lang.String.compareToIgnoreCase(String.java:1220)
at hudson.util.CaseInsensitiveComparator.compare(CaseInsensitiveComparator.java:40)
at hudson.util.CaseInsensitiveComparator.compare(CaseInsensitiveComparator.java:34)
at jenkins.model.IdStrategy$CaseInsensitive.compare(IdStrategy.java:160)
at jenkins.model.IdStrategy.equals(IdStrategy.java:78)
at hudson.model.User.getOrCreate(User.java:393)
at hudson.model.User.getAll(User.java:475)
at hudson.model.User$FullNameIdResolver.resolveCanonicalId(User.java:903)
at hudson.model.User.get(User.java:358)
at hudson.model.User.get(User.java:331)
at hudson.model.User.get(User.java:438)
at hudson.model.Cause$UserIdCause.getUserName(Cause.java:413)
at hudson.model.Cause$UserIdCause.print(Cause.java:423)
at hudson.model.StreamBuildListener.started(StreamBuildListener.java:74)
at hudson.model.Run.execute(Run.java:1725)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:233)

As the stacktrace is about users, I went to the Jenkins users page. Only a limited set is displayed (as expected), but the full list (/asynchPeople) display :
javax.servlet.ServletException: java.lang.NullPointerException
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:778)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:858)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:631)
at org.kohsuke.stapler.Stapler.service(Stapler.java:225)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:96)
at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:58)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:99)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:88)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:48)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:86)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:46)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:610)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:313)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException

Changelog for 1.576 mentions https://issues.jenkins-ci.org/browse/JENKINS-23872 but it seems that there are still problems with users. Any idea on how to fix this problem, and what may be the cause ?