Sadly I have no idea what caused it, as I wasn't close to any trains when it crashed.

Running 0.16.43

Log:
4292.510 Error MainLoop.cpp:1030: Exception at tick 23921357: Error while running event ElectricTrains::on_nth_tick(23)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
ElectricTrains/control.lua:170: in function <ElectricTrains/control.lua:148>
4292.510 Error ServerMultiplayerManager.cpp:96: MultiplayerManager failed: "Error while running event ElectricTrains::on_nth_tick(23)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
ElectricTrains/control.lua:170: in function <ElectricTrains/control.lua:148>"
4292.510 Info ServerMultiplayerManager.cpp:699: mapTick(23921357) changing state from(InGame) to(Failed)

It looks as if the function that handles recharging trains tried to look up a value on a train that got removed. Without knowing more details (was the train blown up? Was it deconstructed? Etc.) I can't really diagnose the issue. Of course, this is two months old now so it's moot, but I'll look out for any other reports of the issue in the future.

Hey,

we are constantly getting the same message on our server, we only have one train running that goes to three stations, two of them are only activated via circuit network.

It's the same message:
2649.056 Error MainLoop.cpp:1035: Exception at tick 31001654: Error while running event ElectricTrains::on_nth_tick(23)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
ElectricTrains/control.lua:170: in function <ElectricTrains/control.lua:148>
2649.056 Error ServerMultiplayerManager.cpp:96: MultiplayerManager failed: "Error while running event ElectricTrains::on_nth_tick(23)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
ElectricTrains/control.lua:170: in function <ElectricTrains/control.lua:148>"

I think I may have narrowed down the issue. Can you upload a copy of the map somewhere for me to experiment with, please?

Yeah, you can add me on discord (NaliLord#8414), I can send it to you, currently it is 36MB.

Do we have a fix for this yet?

I still can't diagnose what's going wrong, so, no. If you can give me any further information, feel free to.

in Control.lua line 170 change:

    chargeTransferValue = math.min(maxFuelValue - currentFuelValue, v[2].energy, maxChargingRate)
    v[2].energy = v[2].energy - chargeTransferValue
    v[4].burner.remaining_burning_fuel = currentFuelValue + chargeTransferValue

to:

    if v[2] ~= nil and v[2].valid then
        chargeTransferValue = math.min(maxFuelValue - currentFuelValue, v[2].energy, maxChargingRate)
        v[2].energy = v[2].energy - chargeTransferValue
        v[4].burner.remaining_burning_fuel = currentFuelValue + chargeTransferValue
    end

Recently I fixed many mod errors myself by adding additional entity nil/valid checks.
I don't know how this checks impact the UPS but at least the crashing stops.