>> The nbackup tool directly opens database file, which means
>> you can't specify server name nor database alias.
>
> First, the manual says something different:
>
> "Instead of a database filename you may also specify an alias."
> http://www.firebirdsql.org/manual/nbackup-backups.html#nbackup-
> backups-full, Section "Full Backup", second comment

Sorry, I was wrong, bad memory ...

But this time I am sure I know the reason -
in firebird.conf you must have:

DatabaseAccess = None

Try Full or Restrict instead.

Ivan
http://www.volny.cz/iprenosil/interbase/