Hello, Rick!

gfix not always can determine such error.

> suggested running a query to check that the correct indexes were there.
> There was no problem with my indexes.

It may be related with corruption of index itself - i.e, it is exists, but
not working.
Try to follow Helen's advice and rebuild indices in this database.

Also, try backup/restore database with -verbose and check output for this
error.
And, may be, some entries in firebird.log exist about it?

Sincerely yours,
Alexey Kovyazin
www.ibsurgeon.com