Are you getting this error message in Transmit:
Host Key Changed for ____
The server’s host key has changed since your last connection. This is most likely due to a server changed, but you should check with your server administrator to be sure. Click OK to accept the new host key.
After clicking “OK” 17 times, I looked to WP-Engine, where I had my site hosted, to help. They got it to work on their machine and told me to figure it out on my own. So I googled and was able to find a fix:
I found a site that said to paste the above code into transmit and it would fix it. Sure enough, I did just that and it worked.
WP-Engine has written an article for this issue: https://wpengine.com/support/updating-host-keys-use-sftp/