This is a known but unfortunately reproduceable error - it only seems to happen for some rare system configurations. I assume that it is caused by a replaced system dll like ws2_32.dll or so, which causes to crash. It could also be caused by a personal firewall or similar running in the background.
The error happens when the server has already disconnected the connection when you press disconnect. The best way to avoid it is to activate FTP keep alive (Ctrl+F - edit connection - keep alive).