Another client also uses the CrushFtp server and he is experiencing the same issue. In this case the server is accessible from the internet. We tried to connect to the server with the latest version of PuTTY (v0.70) using DSA and it also reported the exact same issue.
"Server's host key did not match the signature supplied"
We considered this to be a bug in the CrushFtp server.
The solution is to prefer other host key algorithm. It can be set easily like this:
client.Settings.SshParameters.PreferredHostKeyAlgorithm = SshHostKeyAlgorithm.RSA
Actually, DSA is preferred by default only for backward compatibility. It is suggested to use PreferredHostKeyAlgorithm = SshHostKeyAlgorithm.RSA always in new projects (which most third-party SFTP/SSH clients prefer by default).