When I upgrade from 1.420 to a newer version (including 1.425, just now) and attempt to load Jenkins after restart, I get this stack trace in jenkins.log:

[Winstone 2011/08/12 13:41:49] - Untrapped Error in Servlet
java.lang.NoClassDefFoundError: Could not initialize class org.jvnet.libpam.impl.CLibrary
at org.jvnet.libpam.UnixUser.exists(UnixUser.java:116)
at hudson.security.PAMSecurityRealm.loadUserByUsername(PAMSecurityRealm.java:93)
at org.acegisecurity.ui.rememberme.TokenBasedRememberMeServices.loadUserDetails(TokenBasedRememberMeServices.java:308)
at org.acegisecurity.ui.rememberme.TokenBasedRememberMeServices.autoLogin(TokenBasedRememberMeServices.java:218)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:104)
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 org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
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:66)
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 winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:244)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:150)
at java.lang.Thread.run(Thread.java:662)

Security settings:
Security realm: Unix user/group database
Access control service name: sshd
Authorization: logged-in users can do anything

I did not explicitly log out from the 1.420 install before upgrading.