Symptoms
The first time a custom sftp job runs, it aborts with the following error:
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: read_passphrase: can't open /dev/tty: I/O error
Host key verification failed.
Fix
When connecting for the first time, the command prompt requires user interaction. Connect from the command line manually (in other words, don't use AppWorx), and enter yes when prompted to add the necessary remote machine to the local key hosts file.
After doing this, you'll see the following in the output (compare this to the error above):
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
Warning: Permanently added 'somemachine.edu' (RSA) to the list of known hosts.
debug1: ssh_rsa_verify: signature correct