> > ...
> > gbak: restoring index RDB$FOREIGN42
> > gbak: restoring index RDB$FOREIGN163
> > gbak: restoring index RDB$FOREIGN164
> > gbak: ERROR: invalid request BLR at offset 249
> > gbak: ERROR: data operation not supported
> > gbak: Exiting before completion due to errors
> >
> > The same error occurs even with no-data database (gbak -m).
> > It seems like the error is caused by a particular metadata :
> > if so, how can I determine exactly witch one ?
>
> Does the backup restore successfully to a FB2 server? If not, it may
> be corrupt. If you can restore to FB2, I guess you must be using a
> feature new to FB2 that FB1.5 does not understand (for example,
> indices on expressions). In fact, considering the failure appears to
> occur restoring an index, that is a likely problem.
>
> Can you restore it with the -i switch or does that fail too? To
> downgrade the on disk structure (ODS), you need to make sure that all
> of your DDL is compatible with the lower version of Firebird, then use
> the lower version of gbak against the newer server to generate your
> backup file.
>
> Adam
>

I added -i switch and this is the output:
...
gbak: restoring privilege for user SYSDBA
gbak: restoring privilege for user SYSDBA
gbak: restoring privilege for user SYSDBA
gbak: creating indexes
gbak: ERROR: invalid request BLR at offset 249
gbak: ERROR: data operation not supported
gbak: Exiting before completion due to errors

One way to locate the "incompatible" index is to review last metadata
modification scripts. But it could be helpfull if gbak was able to say
where exactly it's wrong.

In addition, every thing works fine in fb2 to fb2 operations.