Symfony3 makes me logout without me asking

Well, I have this security.yml

# To get started with security, check out the documentation:
# https://symfony.com/doc/current/security.html
security:
    encoders:
      BackendBundle\Entity\.....:
      algorithm: bcrypt
      cost: 4
# https://symfony.com/doc/current/security.html#b-configuring-how-users-are-
loaded
    providers:
        our_db_provider:
            entity:
                class: BackendBundle:......
                property: email
#        in_memory:
#            memory: ~

    firewalls:
        # disables authentication for assets and the profiler, adapt it 
according to your needs
        dev:
            pattern: ^/(_(profiler|wdt)|css|images|js)/
            security: false

        main:
            anonymous: ~
            # activate different ways to authenticate
            form_login:
                login_path: /login
                check_path: /login_check
                username_parameter: _email          
            logout:
                path: /logout
                target: /

            #https://symfony.com/doc/current/security.html#a-configuring-   how-your-users-will-authenticate
            #http_basic: ~

            # https://symfony.com/doc/current/security/form_login_setup.html
            #form_login: ~

    access_control:
        - { path: ^/back, roles: ROLE_ADMIN }

    role_hierarchy:
        ROLE_ADMIN: ROLE_USER
        ROLE_SUPER_ADMIN: [ROLE_ADMIN, ROLE_USER, ROLE_ALLOWED_TO_SWITCH]

and this route:

backoffice_list_paginas:
    path: /back/listPagina
    defaults: { _controller: BackendBundle:Paginas:list } 

When you call that route, everything you do afterwards (calls to other routes) such as this one:

backoffice_list_modificable:
   path: /back/listModif/{idPagina}
   defaults: { _controller: BackendBundle:Modificables:listModif }

called like this:

<a href="{{ path('backoffice_list_modificable',{'idPagina' :  pag.getId() })}}" title="Editar"><i class="pe-7s-scissors"></i></a>

produces a logout with the loss of the user session, appearing the user to null, and then a call to the login function, which produces a redirection to the home without even passing through the controller's function Modifiable: listModif

Any idea why this is happening to me? Thank you very much