After upgrade to 5.1 I get 404

Created by: mikehaertl

I followed the instructions to ugrade from 5.0 to 5.1 and now can't access any gitlab page anymore. In the production.log I see:

ActionController::RoutingError (No route matches [GET] "/gitlab/users/sign_in"):

I've installed gitlab under a relative URL (/gitlab) on my webserver.