Today trying to backup/restore my Firebird 1.5

gbak: restoring privilege for user OPER
IBE: Unsuccessful execution caused by system error that does not
preclude successful execution of subsequent statements.
action cancelled by trigger (1) to preserve data integrity.
could not find column for GRANT.
IBE: Restore completed. Current time: 15:33:38. Elapsed time: 00:04:48

I have made today some alternations to tables.
How can I solve the problem ?
Thanks in advance
Pawel