Port number (either 21 or 22, I guess), is "taken" automatically, Filezilla also fills in with no help from my side. There's server name and in the second step the domain to choose. Directory can be left empty, I guess. That was the case on some of the sites, I uploaded once. Does this help?
Mostly when this error appears, some of the credentials is wrong.
Muse is not displaying an option for port or directory - but by your post, they are not necessary. The one thing that has changed with this site is that we've added an SSL Certificate since the last time I was able to upload through Muse. That could be the problem, BUT since I am able to upload with Fetch, that sorta says it's a problem with Muse.
Click on File > Upload Site to FTP Host
When you get the prompt box to upload to FTP host, go to the Site URL box and simply add an "s" after the http.
Then, hit OK.
Note: You have to make sure the SSL is active on your domain on the server side before you do any of this.
Let me know if the above solution works.