I looked up the error code 30. The result was:
30 Warning Client "%1" no longer responding; connection closed. (%2)
If this is happening to all of your clients at the same time, it may be due to a network issue. Many years ago I had a problem like this that would bring down the entire office. We eventually traced it back to bad switch. Every so often it would drop all connections for a few seconds.
In your case I would check the servers log to see if anything else is showing issues. Also check for loose network cables. If it is network related, it could be the network card in the server, the switch it is connected to or the port on the switch.
Hope this is helpful
Like Bruce is saying; FMS keeps in constant touch with its clients. If the clients stop responding to those "pings" this error will be generated. Could be anything that goes wrong on the network, A very very slow network could potentially also do it. Also make sure that there are no IP address conflicts anywhere on the network.
Also ensure that the computer sleep setting in the System Preferences is set to never.