What a hassle Filezilla was the last few weeks for me. Please read the previous posts if interested in other details about Filezilla I wrote about.
I spent hours of research trying to get this file upload working again. Others suggested if it wasn't a firewall issue that it could be the more current versions. The latest internet research I found to work for FileZilla was FileZilla 3.9.0.6.exe.
FileZilla 3.9.0.6.exe worked flawlessly for the subsim upload. Go to
http://filehippo.com/ for filezilla for this version if needed.
All the internet research suggested for me to use the following ideas to fix FileZilla versions 3.12 or newer.
a) Use SFTP instead of FTP but I did not ask Neal Stevens if this is plausable.
b) Try changing the transfer connection from active to passive etc.
c) Open up Filezilla, then file, then site manager and enter the ftp.subsim information. Then change the encryption stuff under the general tab.
1) Use explicit FTP over TLS if available
2) Require explicit FTP over TLS
3) Require implicit FTP over TLS
4) Only use plain FTP (insecure)
All of these suggestions failed, too.