Thanks for that 
I have looked through the log and it seems to just have entries for:
- login
- feature setup
- file list
- quit
And no errors.
Only odd thing I could find was that after the initial LIST there is a 50 second pause, and then a QUIT command. From the logs, the server has 600s delay before closing the connection, so presumably the client has sent the command.
Was this during an attempted backup?
This is super odd, because there is no other connection attempt apparently made.
If this is correct, then I am leaning towards the issue that @ts678 has uncovered in the thread about too many clients, where it creates two connections: one for listing and one for transfers.
This could explain it if there is something preventing the second connection from the same source IP.
Could you try to start two instances of the FTP client that is working, and see if both instances can connect?