Roundcubemail & subdomain reloaded

[julien.anet at bluewin.ch]

Dear all,
The other post about that subject did not help me any further. 
I am trying to set up a subdomain for roundcubemail (kolab.myserver.com) instead of www.myserver.com/roundcubemail
That's my VHosts conf:
<VirtualHost *:80>
ServerName kolab.myserver.com
DocumentRoot /usr/share/roundcubemail/public_html/
<Directory "/usr/share/roundcubemail/public_html">
<ifModule mod_rewrite.c>
    RewriteEngine On
    # support for CSFR
    RewriteRule ^/[a-f0-9]{16}/(.*) /$1 [PT,L]
    # Be compatible with older packages and installed plugins.
    RewriteCond %{REQUEST_URI} ^/assets/
    RewriteCond %{REQUEST_URI} !-f
    RewriteCond %{REQUEST_URI} !-d
    RewriteRule .*/assets/(.*)$ /$1 [PT,L]
</ifModule>
    Options +FollowSymLinks
    AllowOverride None
    <ifModule mod_authz_core.c>
        Require all granted
    </ifModule>
</Directory>
</VirtualHost>
roundcubemail.conf is inactive in /etc/httpd/conf.d/
My access_log shows 404 for all files which are located under assets/* 
System is a VServer 2 GB with CentOS 7.2
Thanks in advance
J
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