These permissions are identical to the user that works.
backuppc@sirius:~$ ssh -v sirius
OpenSSH_7.4p1 Debian-10+deb9u4, OpenSSL 1.0.2l 25 May 2017
debug1: Reading configuration data /usr/local/BackupPC/.ssh/config
debug1: /usr/local/BackupPC/.ssh/config line 16: Applying options for sirius
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: /etc/ssh/ssh_config line 69: Deprecated option "useroaming"
debug1: Connecting to 192.168.8.3 [192.168.8.3] port 50400.
debug1: Connection established.
debug1: identity file /usr/local/BackupPC/.ssh/sirius_bpc-id_ed25519 type 4
debug1: key_load_public: No such file or directory
debug1: identity file /usr/local/BackupPC/.ssh/sirius_bpc-id_ed25519-cert type -1
debug1: identity file /usr/local/BackupPC/.ssh/sirius_bpc-id_rsa type 1
debug1: key_load_public: No such file or directory
debug1: identity file /usr/local/BackupPC/.ssh/sirius_bpc-id_rsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u4
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.4p1 Debian-10+deb9u4
debug1: match: OpenSSH_7.4p1 Debian-10+deb9u4 pat OpenSSH* compat 0x04000000
debug1: Authenticating to
192.168.8.3:50400 as 'backuppc'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm:
curve25519-sha256@libssh.orgdebug1: 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: Server host key: ssh-ed25519 SHA256:Qmmw9Hw9F68Lv9vTJTHpbJtwOzxUxod4FFAnA16D5bk
debug1: Host '[192.168.8.3]:50400' is known and matches the ED25519 host key.
debug1: Found key in /usr/local/BackupPC/.ssh/known_hosts:1
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering ED25519 public key: /usr/local/BackupPC/.ssh/sirius_bpc-id_ed25519
debug1: Server accepts key: pkalg ssh-ed25519 blen 51
debug1: Offering RSA public key: /usr/local/BackupPC/.ssh/sirius_bpc-id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 535
debug1: No more authentication methods to try.
Permission denied (publickey).
backuppc@sirius:~$