Why does it say "remote host identification failed"?

I got a very strange message when trying to connect to blue! This has never happened before. What does it mean?

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@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
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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:sOPlk68OJZ7lEU87ggLp8zt8RY5AC5R+Lq5+UINMhaU.
Please contact your system administrator.

This happens any time blue's public key changes. It doesn't happen too often. You will need to make the client "forget" blue's old identity. Here is some guidance on how to diagnose and fix.