The text of the error message seems to have changed a little bit, and not for the better.
If you still have access to the old account, make sure it's the domain has been deleted off the manage domains page of the old account. That might help, or might not be possible.
You may also have to contact dreamhost support to free the domain up. Since Dreamhost is not the registrar it won't be a quick solution. Support will basically pass you to the security and abuse team to verify ownership of the domain, once that's done then they can free up the name so that you can add it to your new account.