On October 3rd, MongoLab upgraded our sandbox account to use Mongo 3.0. Great!
Except that we were using an out of date version of the Node driver, which meant that we lost connectivity (with the helpful error message of “auth failed”).
I did a quick push just now to upgrade the driver version, but it’s a major version change and I haven’t looked into the API changes. It doesn’t work — we’ll need to figure out what changed in the mongo driver from 1.4.10 to 2.0.45 (8cd7e73).