Can't connect to my local using devise gem

Can't connect to my local using devise gem

I am working on my local on an app that used to work perfectly. But now, I
get this message error when connecting to my localhost. I use the
devise-2.2.3 gem
Started GET "/users/sign_in" for 127.0.0.1 at 2013-08-12 16:54:42 +0200
Creating scope :closed. Overwriting existing method MergeRequest.closed.
Processing by Devise::SessionsController#new as HTML
Rendered devise/sessions/new.html.haml within layouts/devise (1.8ms)
Rendered layouts/_head.html.haml (26.1ms)
Completed 500 Internal Server Error in 59ms
The I noticed there were no layouts/_head.html.haml. So does somebody has
an idea of the origin of such an error ?