500 Internal server error

For some unknown reason Post view returns 500 Internal server Error
Which I suspect may be .htaccess related

# BEGIN WordPress

Options +FollowSymlinks
Options -MultiViews
RewriteEngine On
RewriteBase /blog/
RewriteRule ^(.*)\.index$ /catch-all.php?var=$1
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]
AddType x-mapp-php5 .php

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . /blog/index.php [L]
# END WordPress

Can anyone identify errors or missing code??

thanks