What NIC is it?

Ah, missed the fact that it's an FA311.

I know that there's nothing wrong with the card itself -- we used 3 of these in the hotel in Ibiza, which has 64 Receivers fitted.

I doubt that it's a problem with NIC/switch interaction -- you'd expect Netgear cards to work properly with Netgear switches, so we can probably discount this as the problem.

It's looking more and more like some kind of port-filter or similar.