Hello!

>And how can this be caused ?

AFAIK, pre-1.5 versions has problem with index garbage collection.
Did you have intensive update/delete/insert activity for table which is
related with index error?

>Can it be due to a restore with a gbak of FB version 1.02 while the server
>has version 1.00 ?

I suppose - no. Gbak slightly changed from the times of Dorothy :)

Sincerely yours,
Alexey Kovyazin
www.ibsurgeon.com
news://news.metadataforge.com/ibsurgeon.eng