Cygwin Windows < Linux rsync - connection unexpectedly closed

Typical - as soon as I post here I resolve the problem (which persisted through dinner and hours of debugging...)

The OpenSSH server that can be enabled as an optional Windows 10 feature doesn't seem to play nice with Cygwin's rsync. I installed OpenSSH through Cygwin and everything connects as expected.

rsync -nva --exclude={"/proc/*","/sys/*","/dev/*","/boot/*","/tmp/*","/run/*","/mnt/*"} -e "C:/cygwin64/bin/ssh.exe -i /cygdrive/h/ssh/BB8.OpenSSH.ppk" [email protected]:/* /cygdrive/h/Users/Matt/Documents/RPi3_Backup