This could really be a sign that “man-in-the-middle” attach is on-going! If you are unsure, check with the system administrator.
If you are reasonably sure that there is no ongoing man-in-the-middle attack, there may be a more mundane reason you are getting this message. This message sometimes occurs when a computer has its OS reinstalled or goes through some other major reconfiguration. You can try running the following command:
ssh-keygen -R “hostname”
After you do so, your computer should treat ‘hostname’ as a new connection and you can try logging in again.
For more information, try consulting https://man.openbsd.org/ssh-keygen.