TLS session resumption for FTP
FolderSync only support this for EdtFTPj/Pro engine. If using Bitvise FTP server there may be incompatability, it is then recommende to use SFTP instead.
Issue connection to FileZilla FTP server
I 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