That message looks completely normal. Our server has quite aggressive inactivity timeouts and will kill the FTP connection if it’s not used for 20 seconds.
To manage your files, you can simply connect to the FTP server, navigate to the right folder, and start the transfer. It doesn’t matter how long the transfer takes, because it’s an inactivity timer, so the transfer will keep going for as long as needed once it’s started.
If you do something else and the connection is killed, just use the buttons at the top to disconnect and reconnect so you can go back to where you left off.
If transferring files does not work, then that could mean there is a problem with setting up the data transfer connection. That could be the result of a network issue on your end. So if that’s the issue, then please try connecting over a VPN.