The sign-up form doesn't seem to allow passwords with non-alphanumeric characters in them. Ironically, when it sees one, it complains that "password must have at least one number and at least one letter" (even though it already does) - I suspect that's just a catch-all message for invalid passwords?

In any case, the restriction makes it incompatible with pretty much all password generators, including the built-in ones in modern browsers. To improve matters, I would suggest dropping all restrictions on what cannot be included altogether.