I have not used Filezilla for a long time but it is not that odd. Even older versions of openssl do not process the long chain properly. openssl is a commonly used program and often integrated with others. The openssl group announced work-arounds for systems that could not update it easily.
Other than Rudy's options, you could make a copy of .../static.managames.com/fullchain.pem and manually remove the last cert (it is the DST Root CA X1). This is making the "short chain" manually. Then use this copy in your FTP TLSRSACertificateFile setting. You will need to edit and copy this each time you refresh the cert.
The 1.12 and later versions of Certbot can create this "short chain" easily but Debian 9 certbot is not easily updated. See this thread.
You could also check Filezilla for updates or their forum for any clues how to tolerate the "long chain".