Transmit FTP App: I Can No Longer Connect Via Secure FTP

I have been using the Transmit app (version 4.4.13) in order to FTP into servers for many years. Recently however, the secure FTP has stopped working on this app (I use FTP with TLS/SSL): on various servers that I log into via FTP, I get the message "Could not connect to server. Could not establish a secure connection."

It's not to do with the server, because I get the same message of servers run by different companies.

I have not changed any preferences in Transmit, nor have I upgraded Transmit from the version I usually use.

So I am guessing this fault may be in macOS.

If anyone has any ideas about what has caused this issue, and how to fix it, I would be most grateful to hear them.