It seems ACLs are more and more used by applications running on Debian rsp RaspberryOS. Unfortunately rsync does complain about ACLs with RC23 when files with ACLs are copied with rsync onto a nfs mounted partition. Today I detected it's possible but you have to use an important option when you mount a partition via nfs.
rsync uses posix ACLs which are supported by nfs version 2 and 3. But they are NOT supported by nfs version 4.
If you mount a nfs partition with following definition in /etc/fstab on a Raspberry
nfsServer:/srv/nfs /backup nfs rw,acl,noauto 0 0
the partition will be mounted with nfs version 4. Just check with mount
nfsServer:/srv/nfs on /backup type nfs4 (rw,relatime,vers=4.2,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp6,timeo=600,retrans=2,sec=sys,clientaddr=::1,local_lock=none,addr=::1)
If you add nfsvers=3 in the fstab
nfsServer:/srv/nfs /backup nfs rw,acl,noauto,nfsvers=3 0 0
this will force the mount to use version 3
nfsServer:/srv/nfs on /backup type nfs (rw,relatime,vers=3,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp6,timeo=600,retrans=2,sec=sys,mountaddr=::1,mountvers=3,mountport=45824,mountproto=udp6,local_lock=none,addr=::1)
and you can successfully copy files with ACLs on the nfs mounted partition.
As an alternative you can remove rsync option -A to copy ACLs from raspiBackup. That way ACLs are not copied. But please test whether a restored system which has no ACLs still runs successfully. See also this FAQ about ACLs.
Unfortunately I wasn't able to get this to work with a Synology.