It might be an error on Microsoft's live side, but the user flow starts from the Minecraft website.

Basically, I started the flow of migrating the Mojang account. After confirming it is really me, it takes it to signup.live website where you have to create a new account. On the view where you have to select birthdate and birthday, clicking next does nothing.

Console has the following error (possibly something that is required has not been passed on?):