ERROR 500 – All existing solutions tested… Please help

Dear guys,
I had a stable wordpress site since more than 2 years. I recently add a second installation on the same FTP server in a subfolder. Let see.

https://mx-4ever.com is the main site
https://mx-4ever.com/mxsimulator is the second one.

I don’t know why, but since I installed the second I can’t access to the first’s dashboard, I get a 500 Error.

I tried to clean my .htaccess, to reinstall wordpress (cleaning then uploading wp-admin and include folders) and disable plugins w/ FTP and w/ phpMyadmin.

I’m pretty sure the plugin “better-wp-security” modified something somewhere. If you want to try, the dashboard link is mx-4ever.com/skynet (instead of wp-admin, using htaccess).

Here is the /www htaccess

SetEnv PHP_VER 5_3
SetEnv REGISTER_GLOBALS 0

# BEGIN Better WP Security
Options -Indexes

<files .htaccess>
Order allow,deny
Deny from all
</files>

<files readme.html>
Order allow,deny
Deny from all
</files>

<files readme.txt>
Order allow,deny
Deny from all
</files>

<files install.php>
Order allow,deny
Deny from all
</files>

<files wp-config.php>
Order allow,deny
Deny from all
</files>

<IfModule mod_rewrite.c>
RewriteEngine On

RewriteRule ^wp-admin/includes/ - [F,L]
RewriteRule !^wp-includes/ - [S=3]
RewriteCond %{SCRIPT_FILENAME} !^(.*)wp-includes/ms-files.php
RewriteRule ^wp-includes/[^/]+\.php$ - [F,L]
RewriteRule ^wp-includes/js/tinymce/langs/.+\.php - [F,L]
RewriteRule ^wp-includes/theme-compat/ - [F,L]

RewriteRule ^login/?$ /wp-login.php?36ahkuyvtjwjdjbxcdi4v [R,L]

RewriteCond %{HTTP_COOKIE} !^.*wordpress_logged_in_.*$
RewriteRule ^skynet/?$ /wp-login.php?36ahkuyvtjwjdjbxcdi4v&redirect_to=/wp-admin/ [R,L]

RewriteRule ^skynet/?$ /wp-admin/?36ahkuyvtjwjdjbxcdi4v [R,L]

RewriteRule ^register/?$ /wp-login.php?36ahkuyvtjwjdjbxcdi4v&action=register [R,L]

RewriteCond %{SCRIPT_FILENAME} !^(.*)admin-ajax\.php
RewriteCond %{HTTP_REFERER} !^(.*)mx-4ever.com/wp-admin
RewriteCond %{HTTP_REFERER} !^(.*)mx-4ever.com/wp-login\.php
RewriteCond %{HTTP_REFERER} !^(.*)mx-4ever.com/login
RewriteCond %{HTTP_REFERER} !^(.*)mx-4ever.com/skynet
RewriteCond %{HTTP_REFERER} !^(.*)mx-4ever.com/register
RewriteCond %{QUERY_STRING} !^36ahkuyvtjwjdjbxcdi4v
RewriteCond %{QUERY_STRING} !^action=logout
RewriteCond %{QUERY_STRING} !^action=rp
RewriteCond %{QUERY_STRING} !^action=register
RewriteCond %{QUERY_STRING} !^action=postpass
RewriteCond %{HTTP_COOKIE} !^.*wordpress_logged_in_.*$
RewriteRule ^.*wp-admin/?|^.*wp-login\.php /not_found [R,L]

RewriteCond %{QUERY_STRING} ^loggedout=true
RewriteRule ^.*$ /wp-login.php?36ahkuyvtjwjdjbxcdi4v [R,L]
</IfModule>
# END Better WP Security

Options All -Indexes

<FilesMatch ^wp-config.php$>
deny from all
</FilesMatch>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

# BEGIN Skynet

SetEnv REGISTER_GLOBALS 0
SetEnv ZEND_OPTIMIZER 1
SetEnv MAGIC_QUOTES 0
SetEnv PHP_VER 5

# Compression

# Activer le filtre
SetOutputFilter DEFLATE

# Certains navigateurs ont des problèmes avec gzip, donc exceptions
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0[678] no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html

# Les images n'ont pas besoin d'êtres compressées puisqu'elles le sont déjà (jpg, gif...)
SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png)$ no-gzip dont-vary

# Pour les proxy
Header append Vary User-Agent env=!dont-vary

And here the /www/wp-admin one

SetEnv PHP_VER 5_3
SetEnv REGISTER_GLOBALS 0

Options All -Indexes

# BEGIN Better WP Security

# END Better WP Security

#AuthUserFile /etc/httpd/.htpasswd
#AuthType Basic
#AuthName “restricted”
#Order Deny,Allow
#Deny from all
#Require valid-user
#Satisfy any

Thanks in advance for ur help !