Viper FTP is able to make FTP sessions using SFTP/SSL protocols. Secure session is first authorized and if done successfully you'll be able to login onto it. Authorization is made by comparison of two RSA keys: one is sent from secure FTP server and the other one - stored on your local machine. To login successfully the keys must be exactly the same. In most cases the RSA key is very stable over time, but for some FTP servers it can change periodically. When the RSA key will change you may not be able to pass the authorization and connection will be refused. Then you must update your RSA key.
The old RSA key is stored on your local machine in “<HOME>/.ssh/known_hosts”. When you try to make connection, and the key on the FTP server has changed you will not pass authorization procedure because the local key is different from present server key. In such a case the best action is to take a short look into FTP Trace just after getting the message "Can't connect to ..." because there should be a short explanation directly from the FTP server. It should be something like "your key is obsolete" or "your key is invalid" etc.
To make a connection you must remove your key stored locally. Go to '<HOME>/.ssh/known_hosts'. If you use only one server you may delete whole file, but when you use more than one server you will lose other entries. In that case you must edit the file (press CTRL-J), find the key and delete it.
Private Keys in SFTP (passphrase)
Viper FTP supports the use of private keys for SFTP servers.
Users may store the private key locally.
If the SFTP server requests the key the user may select the local key and validate it by entering the password.
RSA key & SFTP (passphrase)
Viper FTP User Guide