We checked our web server and FTP server SSL certificates, they are all fine. If you have any problem with our server certificates, please send a screenshot to our support email address.
If you use FileZilla with FTP over SSL, please note FileZilla will display a certificate dialog by default. It is normal and it does not pose any security issue. You just need to click OK. If you don't want to see the certificate dialog again, please check the checkbox "Always trust certificate in future sessions."