Add the following to /etc/ssh/sshd_conf while logged in as root (same password as Admin if you are on a Synology NAS)
| KexAlgorithms diffie-hellman-group1-sha1,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1 Ciphers 3des-cbc,blowfish-cbc,aes128-cbc,aes128-ctr,aes256-ctr |
After adding these, run "ssh-keygen -A" to make it generate the keys not needed before the changes, and restart the SSHd service on the machine (reboot works fine, too)
source: https://www.veeam.com/kb2061
