Please read this
thread regarding that issue. It is a very long thread with many possible fixes for the problem as well as some explanation of what it is. There is no one solution and therefore it will be easier for you to look through that thread and try the various things mentioned rather than have people guess at the exact issue.
In general, though, if it doesn't happen constantly, it really isn't anything to worry about. It's a common error message when you get disconnected for a wide variety of reasons. And it isn't unusual for someone who hasn't had this error often to suddenly start receiving it often on a specific network and not on others. For example, Undernet has been having a variety of issues over the past month or so and it's not unusual to see this error when getting disconnected due to their issues.
The most basic and general explanation is that this error just means you were disconnected unexpectedly and for unknown reason. As you read through that thread, you'll see a variety of possible reasons and probably a more specific definition of the error message. In general, this is caused by some form of network problem. That may be on the server's end or your end or somewhere inbetween. It may be caused by software or hardware (regardless of the wording of the error message). And it can affect only certain programs depending on how sensitive they are to network issues.