Since version 5.7, XData requires the JWT secret to have a minimum size.
To quickly fix this in the demo, just open unit Common.Config and set the JwtTokenSecret to a longer size:
constructor TServerConfig.Create;
begin
AdminPassword := 'admin';
JwtTokenSecret := 'this_should_be_just_a_random_secret';
Urls := [DefaultServerUrl];
end;
Demo will be updated in the next release.