SSH Key Certificates

and the ssh -v:
OpenSSH_8.6p1, LibreSSL 3.3.5
debug1: Reading configuration data /Users/nik/.ssh/config
debug1: /Users/nik/.ssh/config line 666: Applying options for lbp-beta
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 21: include /etc/ssh/ssh_config.d/* matched no files
debug1: /etc/ssh/ssh_config line 54: Applying options for *
debug1: Authenticator provider $SSH_SK_PROVIDER did not resolve; disabling
debug1: auto-mux: Trying existing master
debug1: Control socket "/Users/nik/.ssh/connections/mux_f13923b06c905842fb0135c98f243b441ae95199" does not exist
debug1: Connecting to lbp-beta.snafu.de port 2222.
debug1: Connection established.
debug1: identity file /Users/nik/.ssh/id_rsa type -1
debug1: identity file /Users/nik/.ssh/id_dsa type -1
debug1: identity file /Users/nik/.ssh/id_ecdsa type -1
debug1: identity file /Users/nik/.ssh/id_ecdsa_sk type -1
debug1: identity file /Users/nik/.ssh/id_ed25519 type -1
debug1: identity file /Users/nik/.ssh/id_ed25519_sk type -1
debug1: identity file /Users/nik/.ssh/id_xmss type -1
debug1: certificate file /Users/nik/.ssh/id_rsa_sam-cert.pub type 4
debug1: Local version string SSH-2.0-OpenSSH_8.6
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.4
debug1: compat_banner: match: OpenSSH_7.4 pat OpenSSH_7.0*,OpenSSH_7.1*,OpenSSH_7.2*,OpenSSH_7.3*,OpenSSH_7.4*,OpenSSH_7.5*,OpenSSH_7.6*,OpenSSH_7.7* compat 0x04000002
debug1: Authenticating to lbp-beta.snafu.de:2222 as 'snafu'
debug1: load_hostkeys: fopen /Users/nik/.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: mailto:chacha20-poly1305@openssh.com MAC: compression: none
debug1: kex: client->server cipher: mailto:chacha20-poly1305@openssh.com MAC: compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: ssh-ed25519 SHA256:aeiz4bhAS1fsewpgQ5rdgIvHa7Hs8vW/CiUU+unbKWM
debug1: load_hostkeys: fopen /Users/nik/.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 '[lbp-beta.snafu.de]:2222' is known and matches the ED25519 host key.
debug1: Found key in /Users/nik/.ssh/known_hosts:1752
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: /Users/nik/.ssh/id_rsa_sam-cert.pub RSA-CERT SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU explicit
debug1: Will attempt key: id_rsa_sam RSA SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU agent
debug1: Will attempt key: /Users/nik/.ssh/id_rsa
debug1: Will attempt key: /Users/nik/.ssh/id_dsa
debug1: Will attempt key: /Users/nik/.ssh/id_ecdsa
debug1: Will attempt key: /Users/nik/.ssh/id_ecdsa_sk
debug1: Will attempt key: /Users/nik/.ssh/id_ed25519
debug1: Will attempt key: /Users/nik/.ssh/id_ed25519_sk
debug1: Will attempt key: /Users/nik/.ssh/id_xmss
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/nik/.ssh/id_rsa_sam-cert.pub RSA-CERT SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU explicit
debug1: Server accepts key: /Users/nik/.ssh/id_rsa_sam-cert.pub RSA-CERT SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU explicit
sign_and_send_pubkey: signing failed for RSA "id_rsa_sam": agent refused operation
debug1: Offering public key: id_rsa_sam RSA SHA256:jzuBd+ulgpxou9emJu1RRvIn9bf6plMl0E4mhQLHZvU agent
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic
debug1: Trying private key: /Users/nik/.ssh/id_rsa
debug1: Trying private key: /Users/nik/.ssh/id_dsa
debug1: Trying private key: /Users/nik/.ssh/id_ecdsa
debug1: Trying private key: /Users/nik/.ssh/id_ecdsa_sk
debug1: Trying private key: /Users/nik/.ssh/id_ed25519
debug1: Trying private key: /Users/nik/.ssh/id_ed25519_sk
debug1: Trying private key: /Users/nik/.ssh/id_xmss
debug1: No more authentication methods to try.
mailto:snafu@lbp-beta.snafu.de: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).