Wahaha, I've made a step in my Django's comprehension!
In fact, Django auth module is looking for a view when he asks for: reverse('django.contrib.auth.views.password_reset_done')
But my urls config is namespaces, so Django is lost (here I don't know exactly, maybe someone can explain better).
So I need to say to Django to look first for a non-namespaced url pattern first, just for it's core code.
So my trick is to add a line in the myproject/urls.py:
urlpatterns = patterns('',
url(r'^contracts/', include('apps.contracts.urls', namespace='contracts')),
# this is tricky
# the default auth module need to reverse some urls like reverse('django.contrib.auth.views.password_reset_done')
# but I want to keep a namespaced urls config
# then I need to supply a first url path to django when he look at non-namespaced urls
url(r'^accounts/', include('registration.auth_urls')),
url(r'^accounts/', include('apps.registration_custom.backends.vince.urls', namespace='accounts')),
And everything is working fine!
I admit that this is not an elegant solution, but it has the benefit to keep my namespaces right, so will be the templates.