If you have a valid SSL on your cPanel server but still getting Certificate Expired error
openssl s_client -connect yourserver.com:21 -starttls ftp
Verification error: certificate has expired
Simply restart your FTP server
/scripts/restartsrv_pureftpd
Should be ok then:
openssl s_client -connect yourserver.com:21 -starttls ftp
Verification: OK