Redmine

Hi,
I've a problem to launch my Redmine 3 after an upgrade from 2.6.5
I don't use plugin.

When I launch my unicorn, I've this error :

I, [2015-06-26T11:44:01.279827 #2046] INFO -- : Refreshing Gem list
/home/redmine/www/app/models/comment_observer.rb:18:in `<top (required)>': uninitialized constant ActiveRecord::Observer (NameError)
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/engine.rb:472:in `block (2 levels) in eager_load!'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/engine.rb:471:in `each'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/engine.rb:471:in `block in eager_load!'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/engine.rb:469:in `each'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/engine.rb:469:in `eager_load!'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/engine.rb:346:in `eager_load!'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/application/finisher.rb:56:in `each'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/application/finisher.rb:56:in `block in <module:Finisher>'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/initializable.rb:30:in `instance_exec'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/initializable.rb:30:in `run'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/initializable.rb:55:in `block in run_initializers'
from /usr/lib/ruby/1.9.1/tsort.rb:150:in `block in tsort_each'
from /usr/lib/ruby/1.9.1/tsort.rb:183:in `block (2 levels) in each_strongly_connected_component'
from /usr/lib/ruby/1.9.1/tsort.rb:219:in `each_strongly_connected_component_from'
from /usr/lib/ruby/1.9.1/tsort.rb:182:in `block in each_strongly_connected_component'
from /usr/lib/ruby/1.9.1/tsort.rb:180:in `each'
from /usr/lib/ruby/1.9.1/tsort.rb:180:in `each_strongly_connected_component'
from /usr/lib/ruby/1.9.1/tsort.rb:148:in `tsort_each'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/initializable.rb:54:in `run_initializers'
from /var/lib/gems/1.9.1/gems/railties-4.2.1/lib/rails/application.rb:352:in `initialize!'
from /home/redmine/www/config/environment.rb:14:in `<top (required)>'
from config.ru:4:in `require'
from config.ru:4:in `block in <main>'
from /var/lib/gems/1.9.1/gems/rack-1.6.4/lib/rack/builder.rb:55:in `instance_eval'
from /var/lib/gems/1.9.1/gems/rack-1.6.4/lib/rack/builder.rb:55:in `initialize'
from config.ru:1:in `new'
from config.ru:1:in `<main>'
from /usr/lib/ruby/vendor_ruby/unicorn.rb:48:in `eval'
from /usr/lib/ruby/vendor_ruby/unicorn.rb:48:in `block in builder'
from /usr/bin/unicorn_rails:139:in `call'
from /usr/bin/unicorn_rails:139:in `block in rails_builder'
from /usr/lib/ruby/vendor_ruby/unicorn/http_server.rb:764:in `call'
from /usr/lib/ruby/vendor_ruby/unicorn/http_server.rb:764:in `build_app!'
from /usr/lib/ruby/vendor_ruby/unicorn/http_server.rb:137:in `start'
from /usr/bin/unicorn_rails:209:in `<main>'

Do you have an idea why ?

ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]

My gems :

Using rake 10.4.2
Using i18n 0.7.0
Using json 1.8.3
Using minitest 5.7.0
Using thread_safe 0.3.5
Using tzinfo 1.2.2
Using activesupport 4.2.1
Using builder 3.2.2
Using erubis 2.7.0
Using mini_portile 0.6.2
Using nokogiri 1.6.6.2
Using rails-deprecated_sanitizer 1.0.3
Using rails-dom-testing 1.0.6
Using loofah 2.0.2
Using rails-html-sanitizer 1.0.2
Using actionview 4.2.1
Using rack 1.6.4
Using rack-test 0.6.3
Using actionpack 4.2.1
Using globalid 0.3.5
Using activejob 4.2.1
Using mime-types 2.6.1
Using mail 2.6.3
Using actionmailer 4.2.1
Using actionpack-action_caching 1.1.1
Using actionpack-xml_parser 1.0.2
Using activemodel 4.2.1
Using arel 6.0.0
Using activerecord 4.2.1
Using coderay 1.1.0
Using thor 0.19.1
Using railties 4.2.1
Using jquery-rails 3.1.3
Using mysql2 0.3.18
Using net-ldap 0.3.1
Using protected_attributes 1.1.0
Using ruby-openid 2.3.0
Using rack-openid 1.4.2
Using bundler 1.9.3
Using sprockets 3.2.0
Using sprockets-rails 2.3.2
Using rails 4.2.1
Using rbpdf 1.18.6
Using redcarpet 3.1.2
Using request_store 1.0.5
Using rmagick 2.13.4

Best Regards