In org.apache.catalina.authenticator.AuthenticatorBase.register
(AuthenticatorBase.java:706) a DEBUG log causes a NullPointerException to be 
thrown if the principal object passed to the method is null, for example when 
a previously logged in principal is being logged out.


    protected void register(Request request, Response response,
                            Principal principal, String authType,
                            String username, String password) {

        if (log.isDebugEnabled())
            log.debug("Authenticated '" + principal.getName() + "' with type '"
                + authType + "'");

The acces principal.getName() when principal is null causes an exception. 

Note that is DEBUG traces are not enabled, the processing continues as 
expected.