after client disconnect, server fails with "Attempt to send to not connected connection {1}"

Server sends a lot of update info every frame using sendByChannelToAll using Channels.DefaultUnreliable. This functions splendidly, save that when a client disconnects, very frequently (not invariably) the server halts (is paused in editor) with an

Attempt to send to not connected connection {1}
Failed to send internal buffer channel:1 bytesToSend:475
Send Error: 2 channel:1 bytesToSend:475
ChannelBuffer SendBytes no space on unreliable channel 1

all at...

after client disconnect, server fails with "Attempt to send to not connected connection {1}"