I’m adding this post, mainly for my own reference but thought it may be of interest for others with the same problem.
The problem
Plesk unhelpfully ignores the base url set in the php.ini file with its own settings, but these settings are not configurable from the control panel.
The Solution
Create a “vhost.conf” file for the domain – e.g “/var/www/vhosts/<your.domain.tld>/conf/”
Add the following to the file
<Directory /var/www/vhosts/<your.domain.tld>/httpdocs>
php_admin_value open_basedir none
</Directory>
Then you just need to rebuild the plesk config files and restart apache
/usr/local/psa/admin/sbin/websrvmng -a
/sbin/service httpd restart
Job done 
