ssh, scp works, rsync over ssh doesn't - ssh

Question

I try to run an rsync sync on a Synology from a linux machine.
I have access to the Synology with SSH with key authentication. SSH SCP works, permissions are correct.
~# scp /tmp/foo.bar backupch:/tmp
foo.bar 100% 9 0.5KB/s 00:00
Rsync fails:
rsync /tmp/foo.bar backupch:/tmp -e 'ssh -v'
Rsync output with debug:
OpenSSH_8.9p1 Ubuntu-3, OpenSSL 3.0.2 15 Mar 2022
debug1: Reading configuration data /root/.ssh/config
debug1: /root/.ssh/config line 14: Applying options for backupch
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug1: Connecting to myhost.mydomain [*.*.*.*] port 2222.
debug1: Connection established.
debug1: identity file /root/.ssh/ipdch.backup.key type -1
debug1: identity file /root/.ssh/ipdch.backup.key-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.9p1 Ubuntu-3
debug1: Remote protocol version 2.0, remote software version OpenSSH_8.2
debug1: compat_banner: match: OpenSSH_8.2 pat OpenSSH* compat 0x04000000
debug1: Authenticating to myhost.mydomain:2222 as 'backup'
debug1: load_hostkeys: fopen /root/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ssh-ed25519
debug1: kex: server->client cipher: chacha20-poly1305#openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305#openssh.com MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: ssh-ed25519 SHA256:bbl6asZ3Qc+DnOV8hKPDWdlnJtE2yXMQxb1jyInlc+c
debug1: load_hostkeys: fopen /root/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: Host '[myhost.mydomain]:2222' is known and matches the ED25519 host key.
debug1: Found key in /root/.ssh/known_hosts:1
debug1: rekey out after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 134217728 blocks
debug1: Will attempt key: /root/.ssh/ipdch.backup.key explicit
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,sk-ssh-ed25519#openssh.com,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ecdsa-sha2-nistp256#openssh.com>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Trying private key: /root/.ssh/ipdch.backup.key
Authenticated to myhost.mydomain ([*.*.*.*]:2222) using "publickey".
debug1: channel 0: new [client-session]
debug1: Requesting no-more-sessions#openssh.com
debug1: Entering interactive session.
debug1: pledge: filesystem
debug1: client_input_global_request: rtype hostkeys-00#openssh.com want_reply 0
debug1: client_input_hostkeys: searching /root/.ssh/known_hosts for [myhost.mydomain]:2222 / (none)
debug1: client_input_hostkeys: searching /root/.ssh/known_hosts2 for [myhost.mydomain]:2222 / (none)
debug1: client_input_hostkeys: hostkeys file /root/.ssh/known_hosts2 does not exist
debug1: Remote: /var/services/homes/backup/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug1: Sending environment.
debug1: channel 0: setting env LANG = "en_US.UTF-8"
debug1: Sending command: rsync --server -e.LsfxCIvu . /tmp
debug1: client_global_hostkeys_private_confirm: server used untrusted RSA signature algorithm ssh-rsa for key 0, disregarding
debug1: update_known_hosts: known hosts file /root/.ssh/known_hosts2 does not exist
Permission denied, please try again.
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: client_input_channel_req: channel 0 rtype eow#openssh.com reply 0
debug1: channel 0: free: client-session, nchannels 1
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
Transferred: sent 3412, received 2716 bytes, in 0.1 seconds
Bytes per second: sent 56899.8, received 45293.1
debug1: Exit status 1
rsync error: error in rsync protocol data stream (code 12) at io.c(228) [sender=3.2.3]
Any idea what is going wrong here??

Answer 1

It seems that there is a problem with newer versions of rsync on Synology.
https://community.synology.com/enu/forum/17/post/114547
The solution is to specify the --rsync-path=/bin/rsync with the rsync command.
rsync --rsync-path=/bin/rsync /tmp/foo.bar backupch:/tmp -e 'ssh -v'