Can't connect to MongoDB

Hey guys,
one of our apps suddenly stopped working after a server restart. We get

MongoError: Authentication failed.

The funny thing is that we didn’t change anything and our Express app (Mongoose) can still connect (with the same auth data) to the MongoDB instance. We are using Meteor 1.4 and Mongo v3.2.12. The Mongo instance is also hosted on our own server.

Any idea what could be wrong?

// Edit: Just checked the mongod.log and got the following information:

2017-02-04T15:38:29.533+0100 I ACCESS   [conn64] SCRAM-SHA-1 authentication failed for ouruser on ourdbfrom client ; AuthenticationFailed: SCRAM-SHA-1 authentication failed, storedKey mismatch