[Mark McLeod]

If you think it was an issue with the cRIO Ethernet port, then what were the port lights displaying while the error occurred?

There is a fault that exhibits itself as solid red & amber lights on the cRIO port where it fails to communicate with anyone. It's apparently a lockout condition between the cRIO and the DLink, possibly related to the ARP. Everyone's on but nobody is talking.

Power cycling does nothing to help (it's actually counter-productive in this case), because the timing is such that the order the devices come up seems to cause the lockout.
I've seen it fixed by a simple disconnect/reconnect of the cRIO Ethernet cable followed by a minute of patience. Sometimes a reset of the cRIO alone will clear the condition, but only sometimes.

A sniffer would certainly help diagnose the issue.