I entered a Jenkins Username and left Jenkins Password blank after saving the settings i got that:

HTTP Status 500 -
type Exception report

message
description The server encountered an internal error () that prevented it from fulfilling this request.

exception
javax.servlet.ServletException: java.lang.IllegalArgumentException: Null password was supplied in authentication token
org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:603)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:477)
org.kohsuke.stapler.Stapler.service(Stapler.java:159)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
hudson.plugins.audit_trail.AuditTrailFilter.doFilter(AuditTrailFilter.java:66)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:52)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)

root cause

java.lang.IllegalArgumentException: Null password was supplied in authentication token
org.springframework.util.Assert.notNull(Assert.java:112)
org.acegisecurity.providers.ldap.LdapAuthenticationProvider.retrieveUser(LdapAuthenticationProvider.java:224)
org.acegisecurity.providers.dao.AbstractUserDetailsAuthenticationProvider.authenticate(AbstractUserDetailsAuthenticationProvider.java:119)
org.acegisecurity.providers.ProviderManager.doAuthentication(ProviderManager.java:195)
org.acegisecurity.AbstractAuthenticationManager.authenticate(AbstractAuthenticationManager.java:45)
hudson.plugins.jabber.im.transport.JabberPublisherDescriptor.applyHudsonLoginPassword(JabberPublisherDescriptor.java:272)
hudson.plugins.jabber.im.transport.JabberPublisherDescriptor.configure(JabberPublisherDescriptor.java:575)
jenkins.model.Jenkins.configureDescriptor(Jenkins.java:2513)
jenkins.model.Jenkins.doConfigSubmit(Jenkins.java:2466)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:282)
org.kohsuke.stapler.Function.bindAndInvoke(Function.java:149)
org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:88)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:104)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:561)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:477)
org.kohsuke.stapler.Stapler.service(Stapler.java:159)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
hudson.plugins.audit_trail.AuditTrailFilter.doFilter(AuditTrailFilter.java:66)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:52)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:97)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.19 logs.