_cas_stateful_ gets passed to me as a username from Acegi

[Axel Mendoza Pupo]

This error that you advice is because, you are configuring Acegi app to
act as a proxy authentication by configuring a proxy ticket receptor in
the bean casProxyTicketValidator. This config do an extra authentication
to CAS to authenticate the proxy callback or the others URL, I don't
remember very well this error but I a few months ago I was receiving the
same.
Simply do this:

<bean id="casProxyTicketValidator" class="
org.acegisecurity.providers.cas.ticketvalidator.CASProxyTicketValidator"
>
	<property name="casValidate">
		<value>http://localhost:8080/CAS/proxyValidate</value>
	</property>		
	<property name="serviceProperties">
		<ref bean="serviceProperties"/>
	</property>  
</bean>

If you want to use Acegi as a proxy authentication try to investigate
more about this and I think that you must register your Acagi proxy url
into CAS including another authentication manager or as a registered
service to validate it as a valid service.