You should probably add a little more detail to that question:
Are those error messages reported by the connecting client or in a server log file?
Has the server previously worked correctly or is this a new installation that is failing?
What SFTP client(s) are you using to try access the server?
Are you trying to use key based authentication, password-interactive, or some other form of authentication?
The error "key check failed" could mean that either the client is trying to connect with an invalid key, or that the server is set for key based access only and the client is trying to authenticate using another method (such as "password interactive").
I've never used CoreFTPServer (I used OpenSSH on both Linux and Windows (via cygwin in the case of Windows though there are other ports available too)) but if key based authentication has previously been working and suddenly stopped check the file and directory ownership and permissions for the key files on the server - a common issue people see with OpenSSH is that it will refuse to use keys that are stored in an insecure manner (i.e. in a world writeable directory).
If the client is reporting this error apparently before there has been any attempt to authenticate then it could be reacting to the server's fingerprint not being recognised (i.e. it is a new install or has been changed in some significant way that altered the server's identity). Though I would expect the work "fingerprint" to be used if this was the problem rather than "key", some people get the terms mixed up so the software could be using the wrong term in the error message.