Hi VisualCron, love your work.
I had an SFTP pull setup and it was working fine for months. Then it started to error 90% of the time with the error message (something like) "unexpected response from server".
If I ran the task manually it worked fine so I test the permissions but could not find anything wrong. Google and found a few trouble shooting things that did not help.
With no other good ideas I blamed the SFTP server I was pulling from. I got our clients IT admin to look at their logs, he said that we were not connecting at all when it was failing.
I was about to give up and write a script to do the SFTP pull when I thought of another SFTP task (push) I had setup at about the same time the pull task started failing. I changed the trigger time by a couple of minutes on the SFTP pull task that was failing so the two tasks would not be running at the same time and have not had any problems since.
So ... I think there is a bug that causes one SFTP to fail if two SFTP tasks are run at the same time. We are using version 7.5.1.