Yes, I entered the password. The funny thing is that up till now I didn't have to reenter it as it was saved within the .xar file and, thus, automatically showed whenever I publish the site. I discovered that the same issue also happens with other websites when I click 'Publish'. When I reenter the password, it won't save and I only get the error message.

The last time I updated a website was about a month ago, so I only found out today. I was able to access the server through Magix's FTP utility, so it seems the problem is local.