I found the solution...
According that the subdomain is:
SUB.DOMAIN.TLD
I had to edit the following file:
/var/www/vhosts/DOMAIN/conf/httpd.include
and modify two lines concerning the subdomain:
-----------
After the line:
ServerName SUB.DOMAIN.TLD:80
-----------
I found something like:
-----------
<IfModule sapi_apache2.c>
php_admin_flag engine on
php_admin_flag
safe_mode on
php_admin_value open_basedir "/var/www/vhosts/DOMAIN/subdomains/SUB/httpdocs:/tmp"
</IfModule>
<IfModule mod_php5.c>
php_admin_flag engine on
php_admin_flag
safe_mode on
php_admin_value open_basedir "/var/www/vhosts/DOMAIN/subdomains/SUB/httpdocs:/tmp"
</IfModule>
-----------
Simply switch on to off like that:
-----------
<IfModule sapi_apache2.c>
php_admin_flag engine on
php_admin_flag
safe_mode off
php_admin_value open_basedir "/var/www/vhosts/DOMAIN/subdomains/SUB/httpdocs:/tmp"
</IfModule>
<IfModule mod_php5.c>
php_admin_flag engine on
php_admin_flag
safe_mode off
php_admin_value open_basedir "/var/www/vhosts/DOMAIN/subdomains/SUB/httpdocs:/tmp"
</IfModule>
-----------
Then stop and start Apache... It'll show safe_mode for the local to off with a phpinfo.
And as a dummy, I didn't even notice the option to switch on/off safe_mode for domains in Plesk CP... Especially usefull in grouped operations... Tssss
