It seems to be an error with sign-ups through the app, I had the same issue with several different servers, but was able to create an account on their website and then log in through the app.

Even without it, the server navigation and selection (and no option to migrate your userdata) makes the sign up process a problem not worth solving for many.. A lot of people will just off-hand decide they don't really care, and leave. (This kind of onboarding is not sustainable of we want this fediverse to become a long-term fully fledged Reddit alternative, and not just another Voat with a lower percentage of racists.)

P.P.S: Also, apparently Mastodon was mostly Mark Ruffalo tweets (toots? the logo is an elephant, I think?), which are fine but uninteresting.