>Not related to READWRITE because it does not happen with cursors but only on tables opened with USE SHARED.
>Also, not related to right access, because the problem is not always.. and disappears after reboot. I suspect MDAC.
>But don't understand what exactly happen in memory and why.

MDAC has nothing to do here.
You could install your application on some computer that has no MDAC installed and it will works, just because VFP didn't use any additional drivers to access VFP databases (tables).

Is that application installed on one computer or database resides somewhere on LAN?
Is it possible some other user to LOCK the table (or use it EXCLUSIVE)?
When you get that error can you copy and after that delete some file in the folder where tables are?