OK, finally I found the solution! I hope this might help other users in the future:
1) Install form Softaculos the Oxwall
2) FTP to your site, copy .htaccess file to your local drive and edit as follows
3) Decomment following lines in .htaccess:
RewriteCond %{REQUEST_URI} !^/index.php
RewriteCond %{REQUEST_URI} !/ow_updates/index.php
RewriteCond %{REQUEST_URI} !/ow_updates/
RewriteCond %{REQUEST_URI} (/|\.php|\.html|\.htm|\.feed|\.pdf|\.raw|/[^.]*)$ [NC]
4) Save and upload new .htaccess file to your site
5) Enjoy!!!
In this way everything should work correctly...at least this was a solution for me.
My deepest and kindest regards to calistoy for his precious help! Many thanks!
P.S. Please beg my pardon if my english is quite difficult to understand...