Baseboard heater and thermostat trouble!
I'd be rather surprised if your electric baseboard is actually 120V - that's fairly uncommon. 240V is much more common, without a neutral in normal use (ground, L1 (black) L2 (white, which  should be taped/re-colored red, but often isn't) and 240V between L1 and L2.

Whether the heater is actually 120V or 240V, the circuit is clearly in error if there is only 0.2V between white and black.