>
> With Firebird 2.1.0 it worked OK.
>
> What's going wrong here?
>

If I'm not mistaken everything is as it should be. The ability for non sysdba users to change passwords was added in 2.0. Check the release notes.
I never worked in production with 1.5 series but I think it's possible to modify security database, you get get more details on Ivan Prenosil's site
http://www.volny.cz/iprenosil/interbase/

Sasha