I should point out that we also have a FreeBSD box running FreeNAS with a Chelsio 40GbE card in it. That's connected to the switch via the 2m Tripp-lite cable mentioned above, and is working perfectly with no configuration needed on the switch side. It was auto-detected an the link is active. I tried that same cable with the Mellanox cards and it's not working.
The issue seems to be with either the Mellanox cards and this switch, or with the cables, maybe?