OK, that’s good to know. I suggest doing the following:
1. Measure the resistance between VIN-GND again, and the value should be somewhere around 4.5Kohm to 9Kohm. If it falls below 4Kohm, it’s likely that either there is a short somewhere, or a component is burned (most likely the zener diode, or the LM2596 switching regulator).
2. If the resistance is correct, then something on the 24VAC input line is shorted. There are only a few components on the 24VAC line (fuse, diode D1, capacitor C1, LM2596, so it shouldn’t be too hard to narrow it down.