Since latest firmware also changes the SSH version used by QNAP ( upgraded ) Veeam Backup fails to reconnect to it's backup job ( QNAP Repository ).

Issue is caused by SSH not having the required key exchange protocol by default in the sshd_config

Please perform the following:

# vi /etc/ssh/sshd_config

Add the 2 above lines at any part of the sshd_config

( make sure they are not commented out )

Ciphers aes128-cbc,blowfish-cbc,3des-cbc,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com
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

( Please note the case sensitive and make sure they are 2 single lines without spaces in it, if you copy/paste it will work )

save the sshd_config, restart the ssh service from the Web GUI, and reconnect from Veeam. You will be presented with a request to update the stored key, click confirm and it's back on track.

Reference:

http://blog.millard.org/2014/11/repair-synology-dsm51-for-use-as-linux.html