OK, I was making some wild assumptions.
I thought that the supply to the board would be 5volts.
This idea was re-enforced by an "AD" component with an "04" in its part number which happened to be a 5v TVS.
I agree that the component designation TH1 is a bit strange for a TVS, but to understand why, we would have to get into the head of the guy who designed the thing.
As the component is across the 24v supply, it would make sense that it was either a capacitor or a transient suppressor of some kind.
As the board is now working, unless you can find some more info on "AD04 MK", I would be inclined to find a 30v (ish) Transient Suppressor, fit it to the board and call it fixed.
JimB