Do you hear the squeal (high-pitched whine) of the board? You may need to turn off any trains or what have you.
I realize you tested the board, but my question is does the squeal change pitch when you plug the sign vs. not plugged in. That is, I don't know what the actual voltages are but they are somewhat difficult to accurately measure since it's a high-frequency AC voltage probably 100V or so. A typical hobby multi-meter cannot accurately measure such a high-frequency signal. I only suggest this because it should only take a couple seconds and if the squeal changes, it can indicate that some form of electrical connection is being made to the sign itself.
Presumably you carefully inspected the 3 connections where the gray cable mates to the copper colored flex-cable. Virtually impossible to confirm the 3 connections between the flex-cable and the sign itself. You might try pressure against the tape and sign to see if a loose connection there...though it would be verify difficult to repair (e.g., conductive paint/glue) without damaging the sign. The current is very low so you don't need "0 Ohm" joints.