The action "serverStatus" requires the role "clusterMonitor" on the admin db, see https://docs.mongodb.org/manual/reference/built-in-roles/

Proper fix for this error without giving up security (tested on MongoDB 3.2.1):

use <NodeBB-DB>
db.grantRolesToUser("<NodeBB-User>",[{ role: "clusterMonitor", db: "admin" }]);

Validate by

> db.getUser("<NodeBB-User>")
{
        "_id" : "<NodeBB-DB>.<NodeBB-User>",
        "user" : "<NodeBB-User>",
        "db" : "<NodeBB-DB>",
        "roles" : [
                {
                        "role" : "readWrite",
                        "db" : "<NodeBB-DB>"
                },
                {
                        "role" : "clusterMonitor",
                        "db" : "admin"
                }
        ]
}