DSM 7, DSM 6.

When trying to connect to root@synologymachine it saying

rsync: connection unexpectedly closed (0 bytes received so far)
[sender] rsync error: unexplained error (code 44) at io.c(226) [sender=3.1.1]

You probably do allow rsync service in GUI but it still doesn't work.

You must enable admin account!

Admin account is somehow tied up with root account. When installing DSM, Synology told you it is good idea do disable admin acoount (and from security side it is). But then root on rsync doesn't work.