sub sub directory rewrite problem

Ok, one week on this problem, and I still can’t figure wth.

Root domain : OsCommerce -www.site.com
htaccess =

RewriteCond %{REQUEST_URI} ^/widgets/
RewriteRule .* - [L]
ErrorDocument 404 -http://www.site.com/404.php

everything is ok

Sub Directory : WP -www.site.com/widgets/
htaccess =

RewriteCond %{REQUEST_URI} ^/widgets/sites/
RewriteRule .* - [L]
ErrorDocument 404 -http://www.site.com/widgets/404.php

everything is ok

Sub Sub directory : WP Link Exchange -www.site.com/widgets/sites/
with the link exchange SEO Rewrite, clean urls are :
/category-name/page-number
and the real url is :
index.php?cat=category-name&page=page-number

So my target is to have clean URLS to “link exchange directory” pages like : -www.site.com/widgets/sites/foobar/1

Rewrite Rule is :
RewriteRule ^categories/(.*)/(.*)$ index.php?cat=$1&page=$2 [QSA,L]

The /widgets/sites/.htaccess is (out of the box):

RewriteEngine on
RewriteRule ^categories/(.*)/(.*)$ index.php?cat=$1&page=$2 [QSA,L]
RewriteRule ^subcategories/(.*)/(.*)/(.*)$ index.php?cat=$1&scat=$2&page=$3 [QSA,L]

I add :
ErrorDocument 404 -http://www.site.com/widgets/sites/404.php

I tried all combinations : RewriteBase, slash, no slash, ^/categories, ^categories, index.php, /index.php, etc.

I still got a 404 error when SEO Rewrite is activated, and I (or anyone else) try to reach a -www.site.com/widgets/sites/foobar/1 page

Any idea on that ?