I forgot to mention it is classic server 2.1.

>
> Your database is accessed through the Firebird server: does Firebird
> server have access rights to the database file?
>

It seems that cannot be the problem because after the system is rebooted
everything works fine. The isql program can access the database and
remote programs can also access the database. The database is not
frequently accesses, maybe once or twice a day at the most. Then a batch
job runs gbak locally to backup the database. It will work for a week or
two. Suddenly the backup fail with the Permission denied error. No
permissions have change for the database file but suddenly the database
cannot be accesses by the local user.