Multiplus Not Seeing AC Input Voltage.

Multiplus 24-3000-70-16 is inverting properly as before, but it is suddenly having difficultly with AC input. Test meter on the AC input terminals indicates normal voltage (237V) but the Multiplus reports over the VE BUS a reading of 164 V, 0 Hz. (???) This is of course insufficient for the unit to recognize as valid input power, so it continues inverting for its L1 output, and does not pass through power to AC L2.

The unit worked normally until this started, and no changes to operation of wiring are reported by the system owner.

I can think of no external event that would cause this mismatch between what is measured at the terminals and the voltage that the Multiplus reports. Beyond contacting Victron through the distribution chain for a warranty claim, does anybody have an idea?