When the agent receives commands from the eMake client, they start with code numbers. One such code was not recognized.
Security Exposure: Issuing this error on a connection that requires authentication but has not yet been authenticated should be harmless, because all it tells the attacker is that the command code was not recognized, and what the last command was on that communications channel. The last command is quite predictable before authentication, and so no real information is exposed thereby. Knowledge that a command code is unrecognized would at most provide the agent protocol version, which could be fetched directly by E2A_GET_VERSION, which does not currently require authentication (see CloudBees Build Acceleration Kerberos Authentication for a discussion of that exposure).
Usually eMake and the agent check the version of the protocol that they will use before doing real work, and so only a bug, network glitch, and so on, would cause an unrecognized code to be sent to the agent from the eMake client.