Client disconnections like this are quite common to see in the wild:
Without any additional 'msg' field filled out it's difficult to tell what these are related to, but my suspicion is that they are usually network related issues, i.e IOException trying to read/write to a client socket.
If so, 'Server Error' on its own is a bit misleading, as to me this implies some kind of unexpected internal server problem.
It would be better to have at least something logged in the 'msg' portion of the disconnect so that it's clearer to users what type of problem occurred.
In fact, it seems there are only a small handful of cases where the msg is not passed in with a SERVER_ERROR, and they all have an associated Exception that could be easily added via getExceptionMessage() or similar.