no era un problema di configurazione di apache, come sospettavo... in particolare non riesco a far funzionare mod_mono_auto

così ho dovuto configurarlo a mano (ossia uso mod_mono)
codice:
<VirtualHost *:80>
    	ServerName local.mono
        DocumentRoot /home/nexus6/www/mono/
	AddType application/x-asp-net .aspx
	AddType application/x-asp-net .asmx
	AddType application/x-asp-net .ashx
	AddType application/x-asp-net .asax
	AddType application/x-asp-net .ascx
	AddType application/x-asp-net .soap
	AddType application/x-asp-net .rem
	AddType application/x-asp-net .axd
	AddType application/x-asp-net .cs
	AddType application/x-asp-net .config
	AddType application/x-asp-net .Config
	AddType application/x-asp-net .dll
	DirectoryIndex index.aspx
	DirectoryIndex Default.aspx
	DirectoryIndex default.aspx
	MonoAutoApplication disabled
        AddHandler mono .aspx .ascx .asax .ashx .config .cs .asmx .axd
       MonoApplications "/:/home/nexus6/www/mono"
        <Directory /home/nexus6/www/mono/>		
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Order allow,deny
                Allow from all
                SetHandler mono
        </Directory>
</VirtualHost>