SSH has two protocols that can be used, protocol 1 and protocol 2. More about SSH you can found on Wikipedia.
The older protocol 1 is less secure and if you don't specifically need you should disabled it.
To do thatopen ssh config file:
$ vi /etc/ssh/sshd_config
Change to use only protocol 2
# Protocol 2,1
Save and exit the file.
Restart the ssh service:
$ systemctl restart sshd.service
If you didn't changed the default ssh port yet, you probably want to do that too.