Before the version 4.7.0, WingFTP uses the PKCS#15 format ssh key (provided by cryptlib), but now this format key is not supported, you must use a PEM format RSA key, if you still want to use the old ssh key, then you should convert it into the new format using the tool "p15conv", and select the new key under "Server -> Settings -> SSH host key manager"
You can find this command line tool under the WingFTP's folder, or you can download it here: http://www.wftpserver.com/bbsres/p15conv.zip
Usage: p15conv [sourcefile] [password] [destfile]
Here, [sourcefile] means the old PKCS#15 format key, [password] means the old key password, and [destfile] means the new key path, below is an example:
BTW, if you use the default ssh key "wftp_default_ssh.key" before, then WingFTP will convert it into PEM format automatically, but we highly recommend you generate a new ssh key, because it will be much more secure.