Mac OS X Server user can't change password
According to this old article (10.3!), the lack of a _writers_password property in the NetInfo database would prevent users from changing their password.  That's really outdated, but it seems like a reasonable place to start.

http://support.apple.com/kb/TA21256

(Edit)

From some other sources (as far back as "Running Mac OS X Tiger" page 136) it looks like _writers_passwd is a list of users who can change that particular user's password.  So, setting it to the username should be sufficient.