Hmm, so I wonder if it's the 10/100 part that is solving it, or if it just doesn't like something else about the ubiquity.
its usually not a question of 10 or 100 mbit, but the negotiation of it. I usually suggest 10 mbit as it will be faster than the duet's SD interface and more reliable if there is a compounding issue with less than ideal cabling.
Ubiquiti Dream Pro with SFP+ to a Dell 1Gbit switch (plugged into switch)
so the device is connected to the Dell switch or the dream machine? if the switch that is where the compatibility issue would be most likely, and where disabling auto-negotiation might help.