I'm figuring that GbE is a bit of an edge case at the moment so it's possible it's not as well shaken through in the firmware rather than being a hardware problem - has anyone else experienced any issues with it?
Having had the scope for over 2 years now, I can say with confidence that this has never been an issue with any firmware I have used.
If you are 100% sure about your cable and your setup, then as a last resort I would turn off every single physical device you have that connects to the network. Only after every single device, router, switch, and modem has been turned off: then start turning them back on. This will force the ARP tables to be cleared and rebuilt. It is essential that every single device be turned off, since all of them will store their own tables, and any one of them could be causing the problem (if that is the problem). Sometimes weird things like this problems like this can occur, especially if something else used to be plugged into the same port where the RTB2004 now resides.
Another good candidate to verify is that the IP address is not the same for any two devices. It would probably be a good idea to switch the RTB2004 over to a port on the switch that is known to be good (after you turn everything off, but before you turn them back on). That will help rule out that one port on your switch the RTB2004 is currently using.
Also, try swapping the cable out for a known good cable another device is already using. If you don't have the equipment to test the cable you honestly have no idea if the cable works or not, so using a known good one is essential.
Obviously there could technically be something physically wrong with the RTB2004 hardware port, but it is much safer to assume that it is fine and the problem is in your network.