I made an upgrade from 17.10 to 18.04 about an hour ago. On 17.10 proftpd with TLS and SFTP runs perfekt.
After the Update, Filezilla and WinSCP stopped working with SFTP-Connections, TLS is still working. Filezilla shows:
Befehl: open "user@server" 1111 Error: Server unexpectedly closed network connection
I still can connect with putty/ssh with login and ssh-key.
It's all in the same LAN.
As it's a VM-Machine, i made a snapshot bevore the Update and made the procedure twice with the same result.
I could not find any solution via Google, but installing SFTP with VSFTP.
Best regards for any hint!