Currently, the connection to the server happens when the scenario initializes even if the SFTP module is not the first to launch. This means that the operation done with the SFTP module happens after the start of the scenario, and if before such module runs after some time (for example because some operations are done before), the module could find that the connection timed out. It would be better to establish the connection to the server directly when the module is launched. I reported this issue here: https://make-hq.freshdesk.com/support/tickets/1922803