Hi Yves,
Since the transnit and receive directions of the traffic run on different pins/wires within the cables, it could be possible that the wire carrying the traffic from switch1 to switch2 is not quite running cleanly. That is one reason why you only see errors on one side of the link.
I would suggest that you try a new cable before you do anything else...
Hope that helps - pls rate the post if it does.
Paresh