I’m so sorry, I was automatically trying to use SFTP, FTP works just as fine.
It was working fine on our test servers, that is why I needed the details to properly debug the issue. I found the problem and fixed it of course, it was a very tiny mistake, related to specifically multisite installations with keyword exceptions. It should be fine now.
I will make sure to include this fix in the upcoming release of course, so you don’t have to worry about it.