Not sure if this is the server sending back the wrong message, or the stats script malforming the commend.
Running this command:
/opt/couchbase/bin/cbstats localhost:11210 vkey pkrug 316
Stats 'vkey pkrug 316' are not available from the requested engine.
In 1.8, it would return "not found" because the request is valid, but the key is not there. In 2.0, something changed and now it doesn't actually recognize the command which leads to confusion with the error response.