SSH Key No Longer Working

Hi All,

I hope you can help me with this deeply frustrating issue I'm currently having.

For some reason I cannot connect to any server (including Forge provisioned) or GitHub via SSH with a key.

I am connecting using this command:

ssh [email protected] -i ~/.ssh/id_rsa

(IP address redacted)

When I attempt I get this error:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
SHA256:U0Be52k7G7SJjaLiezTOXR+TSP6lveYtT51o8v8nHc4.
Please contact your system administrator.
Add correct host key in /Users/user/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /Users/user/.ssh/known_hosts:1
ECDSA host key for XXX.XXX.XXX.XXX has changed and you have requested strict checking.
Host key verification failed.

If I delete ~/.ssh/known_hosts I then get this:

The authenticity of host 'XXX.XXX.XXX.XXX (XXX.XXX.XXX.XXX)' can't be established.
ECDSA key fingerprint is SHA256:U0Be52k7G7SJjaLiezTOXR+TSP6lveYtT51o8v8nHc4.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'XXX.XXX.XXX.XXX' (ECDSA) to the list of known hosts.
Received disconnect from XXX.XXX.XXX.XXX port 22:2: Connection blocked because server only allows public key authentication. Please contact your network administrator.
Disconnected from XXX.XXX.XXX.XXX port 22

This happened after I upgraded to MacOS Mojave. However, a coworker is having the same issue connecting and he is still running High Sierra. So I'm doubtful that's the issue.

I've also created a new, separate ssh key and added it in Forge. I continue to get the same errors.

Any ideas?