Your FTP server doesn’t send the whole chain. I guess ProFTPd doesn’t support multiple certificates in one file with the TLSRSACertificateFile directive, as Apache (>2.4.7) does.
You should try to set TLSCACertificateFile to /etc/letsencrypt/live/antonlyap.pp.ua/chain.pem and try again.
Also, keep in mind that Filezilla doesn’t check the CAs that your OS trust so you need to manually trust it in the first connection. No matter whether it is a self-signed certificate, a Let’s Encrypt cert or a commercial one, in all of them, in the first connection, you need to trust the certificate.