/* Utilisateur de la base de données PGSQL. /
define('DB_USER', 'myusername');
`
And here the issue is that when connecting to the PostgreSQL database, it wanted to connect on a database called myusername using the user myusername, instead of trying to connect to the database called mydatabasename with the user myusername.
So to get things working, I had to rename my database from mydatabasename to myusername.
Mentioned in: https://github.com/PostgreSQL-For-Wordpress/postgresql-for-wordpress/issues/73
define('DB_NAME', 'mydatabasename');
/* Utilisateur de la base de données PGSQL. / define('DB_USER', 'myusername'); `
And here the issue is that when connecting to the PostgreSQL database, it wanted to connect on a database called myusername using the user myusername, instead of trying to connect to the database called mydatabasename with the user myusername.
So to get things working, I had to rename my database from mydatabasename to myusername.