SSL Dl fail

SmartFTP connects to the remote FTP site using SSL Implicit option.
Everything is OK. I am able to start the downloading but than in the end I get the following error in the log :
[13:44:51] 475136 bytes transferred. (12.0 KB/s) (00:00:38)
[13:44:51] 226 File sent ok.
[13:44:51] File size mismatch.
[13:44:51] Transfer failed.

The file size is 506743.

Any suggestion?