Child theme displaying default/comments.php? Why?

Hi,

Notes :

• WordPress 2.8.4 local installation
• Custom sandbox child theme
• All plugins disabled
• child theme functions.php disabled

# Background
I’ve made 5-8 themes with WordPress and I am a middle-advanced theme creator (not a complete beginner, not an expert).
For the first time, I’m using sandbox, blueprint, actions and filters.

# Situation
I’m using a child theme (parent : the sandbox default configuration) to create my own theme.
Everything is working fine (header/content/footer customization).
My only problem is that comments_template() is always looking for themes/default/comments.php

# Problem
If my understanding is good, it should first :

1. look for mychildtheme/comments.php
   if not found
      -> 2. look for sandbox/comments.php
      if not found
         -> 3. look for default/comments.php

When mychildtheme activated ( and index.php, functions.php, single.php working fine), the comments section is still displaying default/comments.php
Not even sandbox/comments.php

The thing is : the problem happens even if I copy default/comments.php to mychildtheme and sandbox folders. So, the file comments.php should be valid.

# Diagnostic

• All plugins disabled
• child theme functions.php disabled

# Your opinion?
How can this be possible?
Is there anything else that I should check to ensure compatibility between default/parent/child theme?

Thank you in advance.