Apache error log is quite useful:
mod_fcgid: stderr: PHP Fatal error: Call to undefined function mcrypt_encrypt() in /var/www/vhosts/*********.es/httpdocs/oc-content/plugins/payment/functions.php on line 18
apt-get install php5-mcrypt
Restart web server
Delete all tables starting oc_t_payments_... from the database, oc_t_payments_log last, since there are cascading constraints if a previous installation attempt failed.
Install plugin - no need to download again
Enjoy!