Thanks for trying to help guys.
I could fix this myself, i wanted to share this with you:
I made the database to read_only mode using gfix command and then the backup worked like charm. Then i restored the DB and changed the mode back to read_write!!!

Here is what is there in the firebird.log:
MURTHY_DEV (Server) Thu May 21 17:49:52 2009
Database: C:\data.fdb
internal gds software consistency check (applied differences will not fit in record (177))

MURTHY_DEV (Server) Thu May 21 17:49:52 2009
Database: C:\data.fdb
Dynamic SQL Error
SQL error code = -204
Table unknown
MEMORY_USAGE
At line 1, column 35.

MURTHY_DEV (Server) Thu May 21 19:13:14 2009
Database: C:\data.fdb
Index 1 is corrupt (missing entries) in table PROCESS_STATUS (161)


MURTHY_DEV (Server) Thu May 21 19:13:14 2009
Database: C:\data.fdb
Index 2 is corrupt (missing entries) in table PROCESS_STATUS (161)


MURTHY_DEV (Server) Thu May 21 19:22:20 2009
Database: C:\data.fdb
Page 206348 is an orphan

MURTHY_DEV (Server) Thu May 21 19:26:25 2009
Database: C:\data.fdb
Index 1 is corrupt (missing entries) in table PROCESS_STATUS (161)

--- In firebird-support@yahoogroups.com, Dmitry Yemanov <dimitr@...> wrote:
>
> murthy_prayaga wrote:
> >
> > gbak is failed to back with internal gds software consistency check error.
> > gfix -mend did not fix the errors for some reason.
> >
> > gbak: ERROR: internal gds software consistency check (can't continue after =
> > bugcheck)
> > gbak: Exiting before completion due to errors
> > gbak: ERROR: internal gds software consistency check (can't continue after =
> > bugcheck)
>
> What's inside firebird.log?
>
>
> Dmitry
>