Answered: How to change user/password and fix error:

connect to mysql and do  :

USE ocsweb;

DROP USER 'ocs'@'host';

CREATE USER 'user'@'host' identified by 'password';

GRANT ALL PRIVILEGES ON ocsweb.* TO 'login'@'host';

replace host by your server hostname/ip (usually localhost if ocs is on the same machine than mysql)
replace user by whatever name your want to use
replace password by whatever password you want to use

change OCS Inventory NG config files : 
- in /usr/share/ocsinventory-reports/ocsreports/dbconfig.inc.php,
change those two lines according to what you wrote earlier in mysql :
define("COMPTE_BASE","user");
define("PSWD_BASE","password");

- in /etc/apache2/conf.d/z-ocsinventory-server.conf or /etc/httpd/conf.d/z-ocsinventory-server.conf,
change those two lines according to what you wrote earlier in mysql :
PerlSetEnv OCS_DB_USER user
PerlSetVar OCS_DB_PWD password

restart apache service :
sudo service apache2 restart
or sudo service httpd restart