Light Fixture Junction Box with Remote Switch
When I see an odd voltage like that I think of the two usual causes: (1) ghost voltage reading and (2) floating neutral. 

Ghost voltages are commonly caused by induction from nearby live wires and will disappear when a load is applied. Try connecting a lamp where you had the voltmeter. Try between hot and neutral, between hot and ground, and between neutral and ground. 

Floating neutral would be caused by a break in the neutral somewhere in that panel romex. To test for it switch off all the loads on the same circuit breaker, and then measure the voltages at the panel romex again. 

"Panel" hot to "panel" neutral: 43V, "panel" neutral to ground: 6.5V, "panel" hot to ground: 120V - doesn't really add up, does it? Either the neutral has a varying intermittent bad connection, or something is interfering with your voltage measurement process. You'll have to examine the entire run of the "panel" romex to find the fault.