I don't use FTP much but was trying something simple - connecting to a new site.
I started with Quickconnect and it worked fine but then realised I was supposed to be using FTP over TLS, which is only an option in the Site Manager so, created a new site, setup the options etc. and every time I tried to connect with the Site Manager, I got a 500 error response from the server.
Quickconnect mostly worked but only if I cleared the console first (although that might be a red-herring). I tried switching off TLS in the options but Site Manager would always fail and Quickconnect would mostly work.
I found a similar problem reported a few years ago on the FileZilla forum and they suggested another route. Connect using Quickconnect and then choose File->Copy current connection to Site Manager.
When I did this, it worked fine. I then added the TLS option back in and it still continued working. There was NOTHING different between the connection I created and the one that FZ created when it copied the Quickconnect connection so it must be a bug!
Time to head to FileZilla to report it.