Its seems there is a problem with the 'secure' action (when you don't have the needed credentials) in the sfGuardPlugin. If an app have some credentials defined like that :
credentials : [ Etudiant ]
and if we signin with a user wich not have this credentials, we get an error of redirection > 5, because the 'secure' action is called by the plugin but this one need credentials so it call the secure action but etc...
I think this line sould be added to the security.yml in plugins/sfGuardPlugin/modules/sfGuardAuth/configsecurity.yml :
credentials:  (no credential at all... i don't know if its the good way but it seems to work like that)