Rails - invalid byte sequence in US-ASCII - ruby-on-rails-3

Question

I'm losing all my hair on this:
My rails server starts ok, but whichever the request to it is made (except assets and public content), I get this error :
ArgumentError in HomeController#index
invalid byte sequence in US-ASCII
with this framework trace (no application trace)
better_errors (0.9.0) lib/better_errors/stack_frame.rb:19:in `from_exception'
better_errors (0.9.0) lib/better_errors/error_page.rb:52:in `backtrace_frames'
better_errors (0.9.0) lib/better_errors/middleware.rb:114:in `log_exception'
better_errors (0.9.0) lib/better_errors/middleware.rb:87:in `rescue in protected_app_call'
better_errors (0.9.0) lib/better_errors/middleware.rb:84:in `protected_app_call'
better_errors (0.9.0) lib/better_errors/middleware.rb:79:in `better_errors_call'
better_errors (0.9.0) lib/better_errors/middleware.rb:56:in `call'
actionpack (3.2.13) lib/action_dispatch/middleware/debug_exceptions.rb:16:in `call'
actionpack (3.2.13) lib/action_dispatch/middleware/show_exceptions.rb:56:in `call'
railties (3.2.13) lib/rails/rack/logger.rb:32:in `call_app'
railties (3.2.13) lib/rails/rack/logger.rb:16:in `block in call'
activesupport (3.2.13) lib/active_support/tagged_logging.rb:22:in `tagged'
railties (3.2.13) lib/rails/rack/logger.rb:16:in `call'
actionpack (3.2.13) lib/action_dispatch/middleware/request_id.rb:22:in `call'
rack (1.4.5) lib/rack/methodoverride.rb:21:in `call'
rack (1.4.5) lib/rack/runtime.rb:17:in `call'
activesupport (3.2.13) lib/active_support/cache/strategy/local_cache.rb:72:in `call'
rack (1.4.5) lib/rack/lock.rb:15:in `call'
actionpack (3.2.13) lib/action_dispatch/middleware/static.rb:63:in `call'
railties (3.2.13) lib/rails/engine.rb:479:in `call'
railties (3.2.13) lib/rails/application.rb:223:in `call'
rack (1.4.5) lib/rack/content_length.rb:14:in `call'
railties (3.2.13) lib/rails/rack/log_tailer.rb:17:in `call'
rack (1.4.5) lib/rack/handler/webrick.rb:59:in `service'
/home/augustin/.rvm/rubies/ruby-1.9.3-p448/lib/ruby/1.9.1/webrick/httpserver.rb:138:in `service'
/home/augustin/.rvm/rubies/ruby-1.9.3-p448/lib/ruby/1.9.1/webrick/httpserver.rb:94:in `run'
/home/augustin/.rvm/rubies/ruby-1.9.3-p448/lib/ruby/1.9.1/webrick/server.rb:191:in `block in start_thread'
My team works on windows and it works fine on their platform, but the bug occurs on my computer in ubuntu. Everything worked fine until now, if I rollback to my latest commit it works again, so the error is in the merge.
Questions are:
How can I debug this issue since it's not really verbose?
What does the error actually means? That Ascii is expected and that's not what we have OR that ascii is found and that's not what's expected?
Is the error more likely to be in a gem, in the application controller?
Is there a way to filter all special characters to have an idea where it comes from?
What other info would be usefull to help me debug this?
Readings:
When run bundle get invalid byte sequence in US-ASCII : both solutions offered didn't work for me
is there a way to highlight all the special accent characters in sublime text or any other text editor? : many results, but nothing especially weird
Attached:
Gemfile
source 'https://rubygems.org'
gem 'rails', '3.2.13'
# Bundle edge Rails instead:
# gem 'rails', :git => 'git://github.com/rails/rails.git'
# Database
gem 'pg'
# Database Init
gem 'seed_dump'
# gem "seedbank"
# gem 'active_model_serializers'
# HAML
gem 'haml', '4.0.3'
gem 'html2haml'
# Front-end
gem 'jquery-rails'
gem 'bootstrap-sass', :git => 'git://github.com/thomas-mcdonald/bootstrap-sass.git', :branch => '3'
# gem 'font-awesome-sass-rails'
gem 'font-awesome-rails'
gem 'bootstrap-datepicker-rails'
gem 'jquery-tokeninput-rails' # tag and autocomplete for conversation
# Shared mustache templates for rails 3.
gem 'smt_rails'
# Attachements
gem 'paperclip', '3.4.2'
# Share on Social Network
gem 'social-share-button'
# Jquery Upload File
# gem "jquery.fileupload-rails"
# Map
gem 'mapbox-rails', :git => 'https://github.com/aug-riedinger/mapbox-rails.git'
gem 'leaflet-markercluster-rails'
# Authentication
gem 'bcrypt-ruby', '3.0.1', :require => 'bcrypt'
gem 'devise'
gem 'omniauth'
gem 'omniauth-facebook', "1.4.0"
gem 'oauth2'
gem 'fb_graph'
# Gems used only for assets and not required
# in production environments by default.
group :assets do
gem 'sass-rails', '~> 3.2.3'
gem 'coffee-rails', '~> 3.2.1'
gem 'coffee-script-source', '1.5.0'
gem 'uglifier', '>= 1.0.3'
end
group :development, :test do
gem 'better_errors'
gem 'binding_of_caller'
end
platforms :ruby do
group :development, :test do
gem 'railroady'
end
group :production do
gem 'aws-sdk'
gem 'unicorn'
gem 'newrelic_rpm'
end
end
# Messaging - Notifications
gem 'simple_form'
gem 'mailboxer'
gem 'pusher'
gem 'amistad'
# Pdf generation
gem 'prawn'
gem 'prawnto'
# Payments
gem 'activemerchant'
# SEO
gem 'dynamic_sitemaps'
gem 'metamagic'
# Static files
gem 'markdown-rails'
Thanks for helping a desesperate guy ...

Answer 1

So, I finally found the solution by checking out every commit in the history (git log), testing, and finally making a git diff <commit_hash> <previous_commit_hash> to see what changed.
I had non-ASCII char in a helper (loaded on every call apparently):
def group_currency(group)
currency = group.currency
case currency
when 'EUR'
haml_tag "€"
when 'USD'
haml_tag "$"
when 'GBP'
haml_tag "£"
end
end
So adding this at the beggining of the file solves it:
#encoding: utf-8
module GroupsHelper
...
But what I find more questionning is why can't Rails locate and report in the log the file that rises the problem...
I created an issue on Rails for this: https://github.com/rails/rails/issues/12041