Disabling 243, as suggested in a possibly related bug, did not fix.

JENKINS_JAVA_OPTIONS="-Dhudson.model.User.SECURITY_243_FULL_DEFENSE=false"

Can you describe more about how you reproduce it? I have no idea how to authenticate with AnonymousAuthenticationToken. Please describe how you've configured Jenkins with OAuth and any curl commands or other commands you're using to trigger the error.

Environment details: A Local GitHub Enterprise 2.6.1 install is the auth verification source. Using GitHub Committer Authorization Strategy. Attached screenshot links show settings used for security/auth, and settings used for jenkins github config. (There is a lab install, and a corp install, I have lab for testing).

Jenkins GitHub settings (in case they're relevant):
https://www.dropbox.com/s/xah9mth12ip1e96/GitHuboAuthsettings.png?dl=0
In this link, the generic "GitHub Servers" , there is the lab server used for local authentication, and in "GitHub Enterprise Servers", there are the lab and production servers. (Attempting to remove the lab server from the github servers has no effect.)

Jenkins GitHub in Security:
https://www.dropbox.com/s/ukxais4v5yj7hyz/GitHuboAuthsettings2.png?dl=0
In this link, the lab server is being used for GitHub oAuth, with GitHub Commiter Authorization strategy, with all options allowed save for 'Create Job.."

Longer stacktrace of the error:

java.lang.ClassCastException: org.acegisecurity.providers.anonymous.AnonymousAuthenticationToken cannot be cast to org.jenkinsci.plugins.GithubAuthenticationToken
at org.jenkinsci.plugins.GithubSecurityRealm.loadUserByUsername(GithubSecurityRealm.java:639)
at hudson.model.User$UserIDCanonicalIdResolver.resolveCanonicalId(User.java:1050)
at hudson.model.User.get(User.java:395)
at hudson.model.User.get(User.java:364)
at jenkins.model.Jenkins.getUser(Jenkins.java:2642)
at hudson.plugins.greenballs.GreenBallFilter.mapImage(GreenBallFilter.java:67)
at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:48)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:126)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
....

Before logging in (subset of screen for brevity)
https://www.dropbox.com/s/8i5g21syy2otk7n/imageui1.png?dl=0
After logging in:
https://www.dropbox.com/s/6xv5yjzjc2fu60s/imageui2.png?dl=0

Released 0.24. It should be available in about 8 hrs in the update center. Or download it at https://repo.jenkins-ci.org/releases/org/jenkins-ci/plugins/github-oauth/0.24/github-oauth-0.24.hpi

It handles the exception you experienced I believe. Please confirm for me if the issue is resolved.

Fixes this issue for me. Thanks a lot!

UI elements now showing up correctly when not logged in. WIth 0.24.

Great! Thanks for reporting back everyone.