ArgumentError in Controller#new after updating Rails version

Hello, need some help, App is running under Ruby 1.8.7 and Rails 3.0.11

After updating Rails version to 3.1.11, tsrted to fet strange error on new article page:

Don’t know where this error could come from, as in controller i have only:

class ArticlesController < ApplicationController

def new

@article =


and in view is only form_for @article.

However edit action works fine, as before Rails update.

Any ideas how to fix it?