I got this error message trying to connect to a multiplayer server. I had been in the server but lost connection, so I tried to rejoin.
Before losing my connection, the "Server not responding" began blinking on and off about once a second. That continued until another player paused the game, then left the server. While paused, the "server not responding" progress bar started filling up until I was dropped.
Then I exited Factorio and tried again, but got the message "A player with your name is already in the server"