Hi Graham,

This was certainly one to investigate for the weekend. Where you started investigating from the inside of janusgraph, I started from the user perspective and this is what I did:

1. I replicated your steps on janusgraph-full-0.5.3 and hit the same issue (incorrect username/password)
2. I also replicated your steps on janusgraph-0.3.2 to be sure no bugs were introduced in later versions, but still the same issue
3. I checked the old user list and found https://groups.google.com/g/janusgraph-users/c/iVqlUS2zQbc/m/vmf8PgEQBAAJ  This was interesting: someone had problems with the credentialsDb and only got it working after switching from a Berkeleyje backend to an HBase backend. This was a pattern: your issue also was with Berkeleyje
4. In the authentication section of the gremlin-server.yaml I changed the properties file for the credentialsDb to one using cql-es with a keyspace "credentials" and... remote authentication worked

This was a nasty one, but the effort you had already taken inspired me to do my part. I will make an issue report for this on github.

Best wishes,   Marc