Database running fine...until today. I have reached this delightful error - I'm not sure if MongoLab did an update and the order of authentication is now out of wack with NodeBB, but I'd love some insight. I've tried a number of re-installs and created new DB. Nothing is working.
error: Error creating index not authorized for query on nodebb_west_sand.system.indexes
8/8 19:09  - error: MongoError: not authorized for query on nodebb_west_sand.system.indexes
at Function.MongoError.create (/home/ec2-user/nodebb.2/node_modules/mongodb-core/lib/error.js:31:11)
at queryCallback (/home/ec2-user/nodebb.2/node_modules/mongodb-core/lib/cursor.js:171:34)
at Callbacks.emit (/home/ec2-user/nodebb.2/node_modules/mongodb-core/lib/topologies/server.js:95:3)
at null.messageHandler (/home/ec2-user/nodebb.2/node_modules/mongodb-core/lib/topologies/server.js:246:23)
at Socket.<anonymous> (/home/ec2-user/nodebb.2/node_modules/mongodb-core/lib/connection/connection.js:262:22)
at Socket.emit (events.js:107:17)
at readableAddChunk (_stream_readable.js:163:16)
at Socket.Readable.push (_stream_readable.js:126:10)
at TCP.onread (net.js:538:20)
NodeBB will run perfectly fine on node versions as low as 0.10, there is maybe only a very smaller number of third-party plugins that may require node 4.
You have installed a Node Version Manager called n which does indeed make it easy to change node versions. n does this by copying the files from it's directory to /usr/bin/ when you use it's commands. The reason you had to do this manually probably has to do with a permissions issue.
You are running all your commands as sudo, when none of them should be run with sudo, this can cause permissions issues and is possibly a security risk. Unfortunately, it may be too late to correct this problem. Next time, check here if you run into any issues when not using sudo.