The neutral is not open, there is an impedance someplace in the neutral, there would be no drop across the load if it were open and full voltage would be seen between neutral and ground. There has to be current flow before there can be a drop in voltage.
It is possible the neutral conductor is open but the current has found an alternate path from this point through a poor conductor such as an incomplete or ungrounded metal raceway in contact with earth.
Isolate each section of conductor (between lights, junctions, etc) and meg each neutral to ground to find which section of conductor is defective. If all lights on the circuit are experiencing this the problem is between the first light and the source.