133.438 Error ServerMultiplayerManager.cpp:91: MultiplayerManager failed: "The mod HelicopterRevival (0.2.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event HelicopterRevival::on_configuration_changed
HelicopterRevival/logic/gui/gaugeGui.lua:90: attempt to call method 'removeGaugeGui' (a nil value)
stack traceback:
HelicopterRevival/logic/gui/gaugeGui.lua:90: in function 'destroy'
HelicopterRevival/logic/gui/gaugeGui.lua:7: in function 'reSetGaugeGui'
HelicopterRevival/control.lua:102: in function <HelicopterRevival/control.lua:55>"
133.439 Info ServerMultiplayerManager.cpp:797: updateTick(4294967295) changing state from(CreatingGame) to(InitializationFailed)
133.439 Info CommandLineMultiplayer.cpp:217: Exit point.
133.558 Info ServerMultiplayerManager.cpp:140: Quitting multiplayer connection.
133.558 Info ServerMultiplayerManager.cpp:797: updateTick(4294967295) changing state from(InitializationFailed) to(Closed)
134.533 Info UDPSocket.cpp:218: Closing socket
134.540 Goodbye

When does this happen?

When flying around the server crashed from time to time although no changes were made to the server and now the server doesn't seem to start with your mod anymore due to the error. There have never been any problems with the mod before. What information do you still need?

Since this is a multiplier server, is it possible that one of the helicopters was in flight when the map got saved?

Yes that's right. I've just flown, however this has never been a problem. This is the greatest mod out there and one that plays a big part in our life and I definitely don't want to lose it.

The thing is -- I hadn't tested this on a multiplayer, so I need to take a deeper dive into what's going on...

There had been some small changes in that part of the code, but nothing significantly enough to warrant that kind of crash...

I suspect that this error happened due to incorrect saving in the savegame and am now trying to start the server without the mod and then again with maybe the error has been temporarily fixed. The question, of course, is what exactly is causing the error. I do have programming skills, but unfortunately I don't know the syntax of Factorio Modding. If you need further information of any kind I'm more than happy to help once you've dug deeper into the code lines.

I just had another exact reported, but I'm unable to replicate the issue... I can't make it crash, no matter what I try...

Is there a chance, could be viable to obtain a copy of the map itself?

This bug was actually saved in the save. This was fixed by removing and adding the mod. Maybe that's also an error she doesn't make because it's triggered by Factorio itself when saving through the mod api. Where can I send them the link of the world? I have already uploaded them to Dropbox.