That was it, Roger. The index was corrupted. I dropped it and recreated it, and now everything seems to be working.
Thanks you all for your help.

Quote:
You need to be more precise. How did you "open the message table"?

It's a MSSQLServer feature. You can "show" a table, as Roger described, by simply using a specific GUI option.
Not being a SQL Server expert at all, I too believed that "showing" a table simply meant doing a "select..." .
It seemss that's not the case, instead. While a "select" uses the index (which was corrupted), "showing" a table reads the table directly.

And, thanks a_attrill for pointing to the GUI option to rebuild the index.