All of my backup clients suddenly stopped working. We have been using Duplicati for 3 years now and it has been an excellent backup solution for this (very small) company, and since there was no mention of this issue on the forums I figured it was either our TrueNAS or the firewall. It was the firewall. Not sure if this applies to other firewalls but we use the Meraki MX 85 as our primary and under Security>Threat Protection if you have anything enabled (detection or prevention) with any ruleset enabled you MUST allow the rule for “SSH_EVENT_RESPOVERFLOW - 128:1” in the dropdown. It appears that Meraki doesn’t like the put for SFTP so much and some times the backup runs and others it fails immediately.
Hope this saves someone the 3 days it took me to figure it out.