password is pure rsync protocol so you probably need an rsyncd running in libreelec. Other option is to use rsync/ssh so you need to enable ssh with pka in libreelec.
Create a private ssh key in Omv (certificates) and add the public key in libreelec for root user as described in the wiki. Then select the key from drop down menu.
Make sure the remote url is correct. Refer to the omv wiki for this. A