There is a bug in the way hadoop-nfs parses the reply for a RPC denied message. Specifically, this happens in RpcDeniedReply.java line #50.
When RPC returns a denied reply, the code should not check for a verifier. It is a bug as it doesn't match the RPC protocol. (See Page 33 from NFS Illustrated book).
I would be happy to submit the patch but I need some help to commit into mainline. I haven't committed into Hadoop yet.