Emake failed in its preparations to authenticate itself to the agent, or the agent failed mutual authentication. (This error is detected on the emake side.)
Emake tried to authenticate itself to a particular agent, but failed to do so, or the agent failed to authenticate itself back to the emake client. The error message text gives further details, including the agent principal and Web UI name.
There are any number of reasons that this sort of failure might occur. The most common example is that, under Unix/Linux, the user might have neglected to run Kerberos kinit.
The one reason that is unlikely is that the agent is unhappy with the emake client--that sort of failure is the subject of EC2128.
Check the error text for clues as to which stage of authentication failed. For example, under Unix/Linux, be sure the user has run Kerberos kinit (or the equivalent thereof).