Thanks Mariano
Just power cycled the server, and still getting:
Permission denied (publickey).
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: unexplained error (code 255) at io.c(235) [Receiver=3.1.3]
No doubt the problem is with me, trying to log in as wrong user or something - though I have tried logging in from ReadyNAS using the name I gave that public key when adding it to Digital Ocean, as well as ‘root’. Neither seem to work.