I get this error when a friend tries to log into the server. When I log in to the server, everything is fine.

control.on_player_created
create button 'ion-cannon-button'
80.250 Info ServerMultiplayerManager.cpp:978: updateTick(23436003) received stateChanged peerID(1) oldState(WaitingForCommandToStartSendingTickClosures) newState(InGame)
2025-04-13 20:28:44 [JOIN] Mega387 joined the game
81.136 Error MainLoop.cpp:1449: Exception at tick 23436060: The mod Orbital Ion Cannon (Space Edition) (3.8.9) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event Kux-OrbitalIonCannon::on_nth_tick(60)
Kux-OrbitalIonCannon/modules/gui.lua:296: attempt to index field '?' (a nil value)
stack traceback:
Kux-OrbitalIonCannon/modules/gui.lua:296: in function 'update_GUI'
Kux-OrbitalIonCannon/control.lua:138: in function 'fnc'
Kux-CoreLib/lib/EventDistributor.lua:152: in function <Kux-CoreLib/lib/EventDistributor.lua:148>
81.136 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Orbital Ion Cannon (Space Edition) (3.8.9) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event Kux-OrbitalIonCannon::on_nth_tick(60)
Kux-OrbitalIonCannon/modules/gui.lua:296: attempt to index field '?' (a nil value)
stack traceback:
Kux-OrbitalIonCannon/modules/gui.lua:296: in function 'update_GUI'
Kux-OrbitalIonCannon/control.lua:138: in function 'fnc'
Kux-CoreLib/lib/EventDistributor.lua:152: in function <Kux-CoreLib/lib/EventDistributor.lua:148>"
81.136 Info ServerMultiplayerManager.cpp:808: updateTick(23436060) changing state from(InGame) to(Failed)
81.146 Quitting: multiplayer error.
81.146 Info GlobalContext.cpp:769: Deleting active scenario.

It's not the multiplayer, it's the GUI unifier, have you reinstalled it or started a game with it in general?
It breaks the tool button at the top. and when an update comes, it can't be written because the expected content is missing.

try 3.8.10, i try to overwrite the changes the other mod did

On the server, which is on a Linux machine, everything is fine when I play alone. When any second player connects to the server, this error appears immediately and the server is rolled back for 20 minutes (4 autosaves back).

Now I will check with the new version and write what's up within an hour.

Now I will check with the new version and write what's up within an hour. (**unfortunately my friends aren't playing today, so I can't check.)

Not sure if it's related, but I've started a fresh game with this mod and GUI unifier, and for some reason the panel on the left was available right away, and there was no way to close it, since the button itself appears only after you launch the sattelite with cannon. Disabling this mod, saving the game, then enabling it again fixed the issue. Disabling GUI unifier did nothing. Using the cheat option to spawn a sattelite so the button would appear also worked. It seems to happen only in multiplayer, and not even on every start. Minor issue, just kinda annoying, since you can't close the panel otherwise for a long time.

press I-key to toggle the panel

Ah and yep you fixed that error. Thx, for now me and my friends can play!