In preparation for our our move to fastmail.com, we'll be doing some testing on beta.fastmail.fm. So if you use the beta server, expect some changes and potential issues over the next few days.
Currently that means if you go to beta.fastmail.fm, you'll immediately be redirected to https://beta.fastmail.com. This is expected. Note that you can't currently create @fastmail.com aliases or rename your account to @fastmail.com. This is expected. This will only be available from Thursday as described in the original blog post.