Rails 3: 'The file type yml is not known' when trying to access basic _form.html.erb

I'm trying to write up a very basic rails app, but any time I write up even the simplest form I get the following error:

 I18n::UnknownFileType in Posts#add

Showing /home/john/Websites/sandbox/rails-messing/app/views/posts/_form.html.erb where line #14 raised:

can not load translations from /usr/local/rvm/gems/ruby-head/gems/activesupport-3.0.0.beta4/lib/active_support/locale/en.yml, the file type yml is not known

Extracted source (around line #14):

11:     <% end %>
12: 
13:     <div class='field'>
14:         <%= f.label :title %><br />
15:         <%= f.text_field :title %>
16:     </div>
17: <% end %>

Trace of template inclusion: app/views/posts/add.html.erb

Rails.root: /home/john/Websites/sandbox/rails-messing
Application Trace | Framework Trace | Full Trace

app/views/posts/_form.html.erb:14:in `block in _render_template__2621936652101774794_37048540__1740943893204605353'
app/views/posts/_form.html.erb:1
app/views/posts/add.html.erb:1
app/controllers/posts_controller.rb:27:in `add'

Any ideas what's going on? The locale/en.yml file is there & untouched. The rest of the install seems to work fine. The form is just basic scaffold-generated stuff. Specifically:

<%= form_fo开发者_C百科r(@post) do |f| %>
    <% if @post.errors.any? %>
        <div id='error_messages'>
            <h2><%= pluralize(@post.errors.count, "error") %> prevented this form from being saved:</h2>
            <ul>
                <% @post.errors.full_messages.each do |msg| %>
                    <li><%= msg %></li>
                <% end %>
            </ul>
        </div>
    <% end %>

    <div class='field'>
        <%= f.label :title %><br />
        <%= f.text_field :title %>
    </div>
<% end %>

Any ideas much appreciated - I'm completely stumped.

Just in case anyone has the same problem:

I forgot I was running ruby-pre in RVM, and it seems the latest 1.9.3 pre had problems with the YAML interpreter. I reverted the project to 1.9.2 release candidate (Where it should have been in the first place) and everything was solved. Hope this helps. ;-)