While updating from kerberos-sso 1.4 to 1.6 we encountered a NPE when activating the plugin:

java.lang.NullPointerException
at org.codelibs.spnego.SpnegoFilterConfig.doClientModule(SpnegoFilterConfig.java:179)
at org.codelibs.spnego.SpnegoFilterConfig.<init>(SpnegoFilterConfig.java:138)

Version 1.5 works, and after git bisecting 1.5..1.6 I found the problematic commit to be 9a3cffe: "Add support for JCasC"

Looking at SpnegoFilterConfig.java:179 (specified as v1.0.1 in kerberos-sso's pom)

// we only expect one entry
final AppConfigurationEntry entry = config.getAppConfigurationEntry(moduleName)[0];

There is no changelog entry for 1.6 on the wiki, and I can not find a difference between the provided login.conf and ours, besides principal and keytab filename.