We have two separate buildings, a house and a garage/workshop, about 20 feet apart, sharing the same electrical service. On the service entrance, there's a Siemens whole-house surge protector. The service enters the workshop where it splits up in a large junction box. One part goes to the load center breaker box for the garage/workshop, which has a 100A main breaker. The other part goes through PVC conduit under ground to the load center breaker box for the house, which has a 200A main breaker. The grounding system for the house/garage/workshop combo includes nine 10-foot copper-clad ground rods connected to each other (in a loop around the buildings) by 300 feet of buried 1/0 AWG bare copper wire, and connected to the service entrance with 1/0 AWG bare copper wire.
(It's perhaps worth noting here that the electrical inspector, when passing our electrical for final, told me it was one of the best-wired and best-grounded houses he had ever seen. There's no shoddy work on which to place blame for this weird issue.)
The house's load center is primarily filled with combination AFCI+GFCI circuit breakers. The water heater in the house is a gas tankless and hasn't given us any problems.
The garage/workshop is primarily GFCI breakers (no AFCI or AFCI+GFCI breakers). There's a small mud room in the workshop serviced by a small 56-amp 240-volt (no neutral, just two legs and a ground) electric tankless water heater on a 60-amp circuit.
Sometimes, and not every time mind you, or even most of the time, but sometimes, when I turn on the sink or shower in the workshop mudroom (thus activating the electric tankless water heater), two combo AFCI+GFCI breakers in the house (a separate building, remember) trip. It's always the same two breakers: The circuit for the server/equipment rack in the basement and the circuit for the bedroom receptacles. It's never any other circuits in the house that trip, and it's never any of the circuits in the garage that trip.
It might be worth noting that the circuit for the server/equipment rack has a very large power conditioner / surge protector / battery backup system on it, and the bedroom circuit has surge-protection-built-in receptacles. However, these aren't the only circuits in the house (or even the garage) with surge protection. The (AFCI+GFCI) circuit for the entertainment center in the living room has a surge-protection-built-in receptacle and a battery backup but never trips, and a (GFCI) circuit in the garage/workshop building has a surge protector / battery backup plugged into it and never trips.
I'm at a loss to explain how turning on a tankless water heater in one building can trip (the same two) AFCI+GFCI breakers in another building. Anyone have any thoughts?