Remote backup of IMAP user fails with dsync_remote epoll_ctl error

I’m trying backup a user’s IMAP folders from a system running dovecot 2.1.7. to a system running dovecot 2.3.19.1. After much perusing of the web and Dovecot documentation I tried this:

 1) On the 2.1.7 host set up the doveadm server to listen on TCP port 2425 and defined a doveadm_password value

 2) Issued the following command on the 2.13.19.1 system:

       doveadm -o doveadm_password=<password> backup -R -u <username> -S <host>:2425 tcp:<host>

 3) The command returned the following errors:

        Error: doveadm server disconnected before handshake: EOF
        Error: Command backup failed for <username>: EOF

 4) In the Dovecot log on the 2.1.7 system I found the following error message:

       dsync(<username>): Error: user <username>: Initialization failed: Namespace '': Unknown mail storage driver tcp
       dovecot: dsync(<username>): Fatal: User init failed

A couple of questions:

1) I thought that setting up doveadm-server as an inet_listener (with a password) meant that all communications (including the backup) occurred over that link - is that not correct?

Or does SSH also need to be enabled as well?

2) Am I specifying the arguments correctly? The definition of the destination argument is confusing in the documentation - if doveadm is trying to backup a user’s IMAP mailboxes from a remote source, and you’ve already specified the information needed to connect to the remote sources doveadm-server why would the source need to be specified again via the TCP: argument?