This is the underlying issue here: restic runs ssh to open the connection to the sftp server, and the ssh subprocess died. We need to find out why it was terminated, and resolve this. It has (almost) nothing to do with restic 
It’d be great if you could reproduce the issue, passing the following parameters to get verbose logs from the SSH client:
-o sftp.command="ssh -v -s sftp backupserver"
Maybe we can see something here…