Thanks! According to the FileZilla log, the customer is apparently using FTP over TLS in FileZilla. According the Rebex logs you posted before, they have been using plain unencrypted FTP via Rebex FTP.
FileZilla logs also reveal that "EPSV" command has been used instead of "PASV" to start a passive data connection. With Rebex, you can enforce usage of this command using Settings.ForceExtendedDataChannelCommand option.
Based on this, please try using FTP over TLS with Rebex FTP with this server as well, and try enforcing the EPSV command. If it turns out this solves the issue, we'll consider adding Settings.PreferExtendedDataChannelCommand option that would use EPSV with servers that advertise support for it (which would correspond to FileZilla's behavior).