The 120V goes from the Thermostat/ECO directly to the heating element by way of the black wire on the heating element (usually). If you have 120V (to ground) where the wire from the thermostat attaches to the heating element, and the element tests ok, I would say the return or neutral path must be open. You need a complete circuit for it to work.
With power OFF, disconnect the white lead from the heating element. That's the return wire. Check continuity from that wire (disconnected from the element) to the neutral pin (the long pin) on the nearest outlet (probably in the UDC). This is described with a picture of an outlet, on page 9 of the
Water Heater Troubleshooting Guide.
If you don't have 0 ohms, the next place to investigate would be the junction box on the front, inside corner of the water heater. You'll have to take down the rear wall of the passthrough basement storage next to the UDC to get to it.
Turn off power before touching the box.