The tag "v0.9.3" points to an unstable release of v0.9.3.

git clone https://github.com/NodeBB/NodeBB.git
cd NodeBB
git checkout v0.9.3

results in a commit hash da78e4f... one revision before this issue was fixed.

I had to

git reset --hard 6486197623f8f32ab354503ca40ab6e8e3e4551e

on a fresh checkout to get the same stable "v0.9.3" hash , I got on another machine by first checking out v0.9.2 and then upgrading to v0.9.3.

No issues with Mongo 3.2.1 with this "v0.9.3" revision.

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"
                }
        ]
}