# You could need to un-comment the following line
Options +FollowSymlinks
DirectoryIndex portal.php index.php
RewriteEngine On
RewriteBase /Extreme

#########################################################
# forum SEO REWRITE RULES #
#########################################################
# AUTHOR : dcz http://www.forum-seo.com/
# STARTED : 01/2006
#####################################################
# FORUMS PAGES
########################
RewriteRule ^cherche\.html$ /trouve.html [QSA,L]
# FORUM INDEX (un-comment if used)
RewriteRule ^index\.html$ /index.php [QSA,L,NC]
# FORUM PROTECTION RULE
# RewriteRule ^/[a-z0-9_-]+/([^/]+\.html)$ /index.php [R=301,L,NC]
# CATEGORIES
RewriteRule ^/[a-z0-9_-]*-c([0-9]+)\.html$ /index.php?c=$1 [QSA,L,NC]
# PAGINATED FORUM
RewriteRule ^/[a-z0-9_-]*-f([0-9]+)-([0-9]+)\.html$ /viewforum.php?f=$1&start=$2 [QSA,L,NC]
# FORUM
RewriteRule ^/[a-z0-9_-]*-f([0-9]+)\.html$ viewforum.php?f=$1 [QSA,L,NC]
# PAGINATED TOPIC
RewriteRule ^/[a-z0-9_-]*-t([0-9]+)-([0-9]+)\.html$ /viewtopic.php?t=$1&start=$2 [QSA,L,NC]
# TOPIC
RewriteRule ^/[a-z0-9_-]*-t([0-9]+)\.html$ //viewtopic.php?t=$1 [QSA,L,NC]
# POST
RewriteRule ^/post([0-9]+)\.html$ /viewtopic.php?p=$1 [QSA,L,NC]
#PROFILES
RewriteRule ^/membre([0-9]+)\.html$ /profile.php?mode=viewprofile&u=$1 [QSA,L,NC]
# END forum PAGES
#####################################################
#########################################################
# GYM SITEMAPS AND RSS REWRITE RULES #
#########################################################
# AUTHOR : dcz http://www.forum-seo.com/
# STARTED : 2006/02/22
########################
# RSS main
RewriteRule ^Extreme/rss-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?$1&$2 [L]
# RSS forums
RewriteRule ^Extreme/forums-rss-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?forum&c&$1&$2 [L]
# RSS all
RewriteRule ^Extreme/([a-zA-Z0-9_-]+)-rss([0-9]*)-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?$1=$2&$3&$4 [L]
# RSS forum topics
RewriteRule ^Extreme/.+-rf([0-9]+)-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?forum=$1&$2&$3 [L]
# SitemapIndex
RewriteRule ^Extreme/sitemaps\.(xml(\.gz)?)$ /Extreme/sitemap.php [L]
# Sitemap modules
RewriteRule ^Extreme/([a-zA-Z0-9_-]+)-sitemap\.(xml(\.gz)?)$ /sitemap.php?$1 [L]
# Forum Sitemaps
RewriteRule ^Extreme/.+-gf([0-9]+)\.(xml(\.gz)?)$ /sitemap.php?forum=$1 [L]
# Yahoo! urllist.txt
RewriteRule ^Extreme/urllist\.(txt(\.gz)?)$ /urllist.php [L]
#########################################################
# END GYM SITEMAPS AND RSS REWRITE RULES #
#########################################################

# You could need to un-comment the following line
Options +FollowSymlinks
DirectoryIndex portal.php index.php
RewriteEngine On
RewriteBase /Extreme

#########################################################
# forum SEO REWRITE RULES #
#########################################################
# AUTHOR : dcz http://www.forum-seo.com/
# STARTED : 01/2006
#####################################################
# FORUMS PAGES
########################
RewriteRule ^cherche\.html$ /trouve.html [QSA,L]
# FORUM INDEX (un-comment if used)
RewriteRule ^index\.html$ /index.php [QSA,L,NC]
# FORUM PROTECTION RULE
# RewriteRule ^Extreme/[a-z0-9_-]+/([^/]+\.html)$ /index.php [R=301,L,NC]
# CATEGORIES
RewriteRule ^Extreme/[a-z0-9_-]*-c([0-9]+)\.html$ /index.php?c=$1 [QSA,L,NC]
# PAGINATED FORUM
RewriteRule ^Extreme/[a-z0-9_-]*-f([0-9]+)-([0-9]+)\.html$ /viewforum.php?f=$1&start=$2 [QSA,L,NC]
# FORUM
RewriteRule ^Extreme/[a-z0-9_-]*-f([0-9]+)\.html$ viewforum.php?f=$1 [QSA,L,NC]
# PAGINATED TOPIC
RewriteRule ^Extreme/[a-z0-9_-]*-t([0-9]+)-([0-9]+)\.html$ /viewtopic.php?t=$1&start=$2 [QSA,L,NC]
# TOPIC
RewriteRule ^Extreme/[a-z0-9_-]*-t([0-9]+)\.html$ //viewtopic.php?t=$1 [QSA,L,NC]
# POST
RewriteRule ^Extreme/post([0-9]+)\.html$ /viewtopic.php?p=$1 [QSA,L,NC]
#PROFILES
RewriteRule ^Extreme/membre([0-9]+)\.html$ /profile.php?mode=viewprofile&u=$1 [QSA,L,NC]
# END forum PAGES
#####################################################
#########################################################
# GYM SITEMAPS AND RSS REWRITE RULES #
#########################################################
# AUTHOR : dcz http://www.forum-seo.com/
# STARTED : 2006/02/22
########################
# RSS main
RewriteRule ^Extreme/rss-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?$1&$2 [L]
# RSS forums
RewriteRule ^Extreme/forums-rss-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?forum&c&$1&$2 [L]
# RSS all
RewriteRule ^Extreme/([a-zA-Z0-9_-]+)-rss([0-9]*)-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?$1=$2&$3&$4 [L]
# RSS forum topics
RewriteRule ^Extreme/.+-rf([0-9]+)-?(l|s)?-?(m)?\.(xml(\.gz)?)$ /rss.php?forum=$1&$2&$3 [L]
# SitemapIndex
RewriteRule ^Extreme/sitemaps\.(xml(\.gz)?)$ /Extreme/sitemap.php [L]
# Sitemap modules
RewriteRule ^Extreme/([a-zA-Z0-9_-]+)-sitemap\.(xml(\.gz)?)$ /sitemap.php?$1 [L]
# Forum Sitemaps
RewriteRule ^Extreme/.+-gf([0-9]+)\.(xml(\.gz)?)$ /sitemap.php?forum=$1 [L]
# Yahoo! urllist.txt
RewriteRule ^Extreme/urllist\.(txt(\.gz)?)$ /urllist.php [L]
#########################################################
# END GYM SITEMAPS AND RSS REWRITE RULES #
#########################################################

# You could need to un-comment the following line
Options +FollowSymlinks
DirectoryIndex portal.php index.php
RewriteEngine On
RewriteBase /

#########################################################
# forum SEO REWRITE RULES #
#########################################################
# AUTHOR : dcz http://www.forum-seo.com/
# STARTED : 01/2006
#####################################################
# FORUMS PAGES
########################
RewriteRule ^cherche\.html$ /Extreme/trouve.html [QSA,L]

mm tu n'aurais pas tout simplement un index.html physique dans le même dossier ?

Tu as essayé d'autres url que l'index du forum ?

l n'y a pas de raisons qu'une rewriterule marche et aucune autre.