Setup of SFTP Connector - Error: Connection lost due to error 96273

We have problem to setup a sftp connector. We get the error “Connection lost due to error 96273”.

Someone who has used this connector and know how to do the setup?

I have done it a few times. An example here below. It sounds like something networky or something like that.
Does the log show anything more? Any proxies or firewall in between?
Can you set up a sftp-client on the flow server to test that comms are up and running?