mongodb authentication failed code 18

If you are receiving this error and are 100% sure you are using the correct DATABASE username/password. try to update your mongodb client

PHP
pecl upgrade mongodb

Grails
add to to the BuildConfig.groovy file

compile "org.grails.plugins:mongodb:5.0.0.RC1"

Also if you are having problems serializing a list in Grails and only receiving the first item. It may be because <Domain Class>.list() does not return a list or array item but a MongoDB collection now (since MongoDB updates) to remedy this for now I have cast the lists to Arrays.