blank page after an avatar plugin install

I've tried to install avatar plugin.

I've uploaded all the files and I modified the .htaccess file and now the site doesn't work anymore. At the wpmu address is a blank page. I restored the old .htaccess file but no change. I tried many tutorials/fix for this kind of problem. Nothing doesn't apparently work.

Please somebody from here if you know the issue.

The site is

the .htaccess file at the moment of writing:

RewriteEngine On
RewriteBase /</p>
<p>#uploaded files
RewriteRule ^(.*/)?files/$ index.php [L]
RewriteCond %{REQUEST_URI} !.*wp-content/plugins.*
RewriteRule ^(.*/)?files/(.*) wp-content/blogs.php?file=$2 [L]
RewriteRule ^(.*/)?avatar/(.*) wp-content/avatar.php?file=$2 [L]</p>
<p># add a trailing slash to /wp-admin
RewriteCond %{REQUEST_URI} ^.*/wp-admin$
RewriteRule ^(.+)$ $1/ [R=301,L]</p>
<p>RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule . - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-.*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]</p>
<p><IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off