2.4.0 had a number of bugs with subscriber templates getting out of whack due to any number of subscriber add issues. You can try running dbWalker off AnswerMonkey.net and see if it flags any issues. It wont fix them for you automatically but it'll point out where things have problems.
Other stuff I remember coming up where situations where Unity was setup to generate a default NT password with fewer digits than the sites minimum NT password for their domain. Saw a number of those. I'd be willing to bet, however, that you subscriber template(s) are fouled up and the subscriber add/import mechanism is choking as a result. If that's the case we can rebuild your default templates using ConfigMgr. Check and see what dbWalker has to say first...
Jeff Lindborg
Unity Technical Lead/Answer Monkey
Cisco Systems
lindborg@cisco.com
http://www.AnswerMonkey.net (new page for Unity support tools and scripts)