Wait...

I got something (I mean, apart the private messages we exchanged)...

Remove the following line in edit_orders.php

Code:
$language = (int)$_SESSION['languages_id'];
The images are back on the Edit Orders page! Plus, no more crash when you click on "Add product"...

The only bug left is an SQL error on Production Addition :

Code:
1054 Unknown column 'french' in 'on clause'
in:
[SELECT products_name, p.products_id, categories_name, ptc.categories_id FROM products p LEFT JOIN products_description pd ON (pd.products_id=p.products_id AND pd.language_id= french) LEFT JOIN products_to_categories ptc ON ptc.products_id=p.products_id LEFT JOIN categories_description cd ON (cd.categories_id=ptc.categories_id AND cd.language_id= french) ORDER BY categories_name]
If you were entering information, press the BACK button in your browser and re-check the information you had entered to be sure you left no blank fields.
Obviously, the correct term should be cd.language_id=2 (in my case), which brings 2 questions :

- why does the definition of $language cause trouble on this page (no image, wrong links, blank pages) ?
- how to pass the language id only in the SQL statements...

OK, time to go to bed again !!

sanji