Connection via FTPS fails with one of the following errors (depends on FTP client):
Insecure server, it does not support FTP over TLS.
534 Policy denies SSL
Local policy on server does not allow TLS secure connections
Log in to Plesk.
Go to Tools & Settings > Security Policy. Set the Allow both secure FTPS and non-secure FTP connections option and press the Apply button. Set the Allow only secure FTPS connections option back and press the OK button.