Mike
looks to me to be neutral connection ahead of the main breaker. Meter base or at the utility connection or one of their connections closer to the transformer. To date in 31 years all unbalanced voltages between 1 phase to neutral and the other phase in neutral has always been an open or resistive connection at some neutral connection. Once you have verified all the connections you are allowed to fix it then must be further upstream.
I'd get permission or have the utility break the meter seal and check the insides of the meter base.
I assume you have checked all the neutrals accessible to you?
