TLS session resumption for FTP
FolderSync only support this for EdtFTPj/Pro engine. If using Bitvise FTP server there may be incompatibility, it is then recommended to use SFTP instead.
Issue connection to FileZilla FTP server
It would seem the default self-signed certificate for FTPS Explict connection in FileZilla is not compatible with FolderSync. You can create a compatible X.509 certificate using OpenSSL with this command:
openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -sha256