It can be switched INSIDE equipment, but not the building. I only installed one thing ever that had a switched neutral breaker on the front of the equipment.
90A 3 phase 208 with neutral and ground. That was fun. The thing had hidden connectors, but all the wires were cut including the 15 kV at 2A. They had bets i would never get it working. Bought used.
I agree neutral is open look for a corroded wire At the box or ground rod . That cAn cause weird proublems depending on temp humidity etc. sometimes makes connection sometimes not. Coastal areas are the worst with salt etc. I agree unplug everything until you find the proublem for sure