>>What's error number in this case?
>
>That's an interesting one.
>
>The message is "Index file "d:\www\foxpro\reply.cdx" is corrupted. Please rebuild it.".
>
>The error number is 2066.

As Aleksey explained in Re: Error 2066 (undocumented) Index File xxx is corrupted... Thread #810798 Message #811607 it may not be even index corruption. He also gave some suggestions on how to avoid this error.
Tell you truth, I belive that it's a bug in VFP because VFP should handle this situation internaly not generate an error. I had to move one of my applications back to VFP7 because of this error.