How do you connect? Using "QuickConnect" or using the "Site Manager"?

Try to use the site manager. Create a new site, then set the Encryption to "Use Only Plain FTP"

That should allow you to connect.
But even better, tell the FTP server owner to enable FTP over TLS. That is much more secure.

 

That yields identical error as before.

I think this has something to do with ports, which I know near nothing about. For some reason, I never ran into this with WSFTP

 

And you saved that site in Site Manager with that "Plain FTP"? That should not use the TLS.
Also make sure the protocol is FTP. Not any of the other options.

You can also try, under "Transfer" in the site manager, to change the connection from Default to active to passive. All though I don't think that'll make a difference.

Do you know which FTP server is on the other end? Is it IIS or something else?

 

Found this : https://wiki.filezilla-project.org/FTP_over_TLS

And this :

"Normal FTP uses port 21(TCP/UDP) for control and port 20(TCP/UDP) for data. FTP over TLS (FTPS) uses port 990(TCP/UDP) for control and port 989(TCP/UDP) for data. You will need to open both as FTPS prevents the router from detecting which port was negotiated for the data transfer."

 

"Normal FTP uses port 21(TCP/UDP) for control and port 20(TCP/UDP) for data. FTP over TLS (FTPS) uses port 990(TCP/UDP) for control and port 989(TCP/UDP) for data. You will need to open both as FTPS prevents the router from detecting which port was negotiated for the data transfer."

@lunarlander . With all due respect. I think you got this a bit mixed up. The OP is, as far as I can tell, trying to use the Filezilla client to connect to a FTP server that he/she previously connected to with no problem. The OP does obviously not need to open any ports as it worked previously. The issue is with the latest Filezilla client. And ultimately with the way the FTP server is configured.

That being said; The OP needs to connect using the "simple" FTP option. Without using any encryption/security. It's a known issue with the latest Filezilla servers/clients. The client will default to a secure connection which most basic FTP servers aren't set up for. Ideally the server people should enable FTP over TLS. But that's a whole other issue.

As previously stated. The OP needs to use the Site manager to manually configure how to connect. Not use the "Quickconnect" option.