This error could occur for several reasons:

Most likely the connection between the client and server was terminated/closed due to random noise on the line or a timeout. 

Also, it could be that (if they were using host keys) one of the parties didn't like the other parties' host key and dropped the connection, or the client was looking for an authentication type Cornerstone does not support. Cornerstone supports password and pubkey; if the client only supports keyboard-interactive it would disconnect the session after it asks Cornerstone what type of authentication it supports.