invalid byte sequence in US-ASCII during assets:precompile - ruby-on-rails-3

Question

I can no longer deploy my app because of an UTF error, here is the trace :
/srv/outils/www/railsapp/releases/20140414151455$ cd -- /srv/outils/www/railsapp/releases/20140414151455 && RAILS_ENV=prodtest RAILS_GROUPS=assets bundle exec rake assets:precompile --trace
rake aborted!
rake aborted!
invalid byte sequence in US-ASCII
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/trace_output.rb:16:in `block in trace_on'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/trace_output.rb:14:in `map'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/trace_output.rb:14:in `trace_on'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:340:in `trace'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:187:in `display_error_message'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:174:in `rescue in standard_exception_handling'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:165:in `standard_exception_handling'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:93:in `load_rakefile'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:77:in `block in run'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:165:in `standard_exception_handling'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/lib/rake/application.rb:75:in `run'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/gems/rake-10.1.0/bin/rake:33:in `<top (required)>'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/bin/rake:23:in `load'
/srv/outils/www/railsapp/shared/bundle/ruby/1.9.1/bin/rake:23:in `<main>'
The LANG variable is set to fr_FR.UTF-8.
What bother me is that which ruby gives me :
/home/admin/.rvm/rubies/ruby-1.9.3-p286/bin/ruby
I installed a rvm gem and now i have exactly the same error with the correct RVM path.
I tried :
Adding #encoding UTF-8 everywhere
Added :
if RUBY_VERSION =~ /1.9/
Encoding.default_external = Encoding::UTF_8
Encoding.default_internal = Encoding::UTF_8
end
Checked $LANG on my server
Removed any éè etc.. from my previous commits
EDIT : Migrated from ruby 1.9.3 to 2.1
Where to go now to investigate this problem?