php-fpm apache2 403 error on .css .js files - apache

Question

I'm trying to setup an apache2 server on debian jessie with multiple vhosts. I want each one of them to have a different FastCgiExternalServer. I'm using the php version from php5-fpm package on jessie with default /etc/php5/fpm configuration files.
Basic apache configuration is working fine :
<VirtualHost *:80>
ServerName lalala.org
DocumentRoot "/path/to/app/www"
<Directory "/path/to/app/www">
Options Indexes FollowSymLinks MultiViews
AllowOverride None
Order allow,deny
allow from all
Require all granted
</Directory>
FastCgiExternalServer /path/to/app/www -socket /var/run/php5-fpm.sock
AddHandler php-fcgi .php
Action php-fcgi /path/to/app/www
</VirtualHost>
However, I got 403 errors on .css, .js files. I read this article : http://whocares.de/fastcgiexternalserver-demystified/8/, but even with this configuration :
FastCgiExternalServer /path/to/app/www -socket /var/run/php5-fpm.sock
AddHandler php-fcgi .php
Action php-fcgi /virtualpath
Alias /virtualpath /path/to/app/fcgi-uri
(/path/to/app/fcgi-uri is a symlink to /path/to/app/www)
It doesn't work. It's as if I had a local problem but can't figure out what.
Thanks
EDIT :
I found a solution that makes fastcgi execution work :
<FilesMatch \.php$>
SetHandler "proxy:unix:/path/to/sock/socket.sock|fcgi://localhost"
</FilesMatch>
But I think this is more a workaround than a solution.