>I got often a weird error message when trying to do a REPLACE (or INSERT or any table update command) on any table opened in SHARED mode and without NOUPDATE clause at all. I don't understand why this kind of error occurs because we have no table opened in read-only mode. It can happen anywhere (on any table) and anytime. I found that a simple reboot seems to fix that. Do you think it could be related to MDAC (Microsoft Data Access Component) ? Because I have already read something about that on a forum

That could happens if you try to update some fields in the cursor created with SELECT statement w/o READWRITE clause at the end.
By default all cursors created with SELECT - SQL are read-only.

Other thing that could cause that error is that user have only READ access to the folder where tables are placed.
I can't remember some troubles with MDAC, because VFP didn't use it when it access its own tables, but you could search it.