Answer by A.Nelson for Issue Connection to server using Remote-FTP atom package and Private Keys

The problem is the backslashes. The .ftpconfig file is in JSON format and slash followed by character has a special meaning. To use your ssh key you just have to escape the slashes. In other words change :

C:\Users\user\.atom\server\putty_private_key_sbappsupconvrted.ppk

To :

C:\\Users\\user\\.atom\\server\\putty_private_key_sbappsupconvrted.ppk

Then it will work with no issues.