during an attack (maybe one of the balancers got destroyed):

The mod Belt Balancer 2 (2.0.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer-2::on_entity_died (ID 4)
belt-balancer-2/objects/balancer.lua:280: Arguments count error for 'spill_item_stack': Expected 1 argument but 4 were given
stack traceback:
[C]: in function 'spill_item_stack'
belt-balancer-2/objects/balancer.lua:280: in function 'empty_buffer'
belt-balancer-2/objects/balancer.lua:258: in function 'check_track'
belt-balancer-2/objects/part.lua:313: in function 'remove'
belt-balancer-2/control.lua:174: in function <belt-balancer-2/control.lua:163>

Getting the same error here, i also find I get the same crash if I delete a rare balancer it crashes my game

This has been fixed now.

As of version 2.0.6 it doesn't seem to be fixed

5929.695 Error MainLoop.cpp:1429: Exception at tick 3389380: The mod Belt Balancer 2 (2.0.6) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer-2::on_entity_died (ID 4)
belt-balancer-2/objects/balancer.lua:280: Arguments count error for 'spill_item_stack': Expected 1 argument but 7 were given
stack traceback:
[C]: in function 'spill_item_stack'
belt-balancer-2/objects/balancer.lua:280: in function 'empty_buffer'
belt-balancer-2/objects/balancer.lua:258: in function 'check_track'
belt-balancer-2/objects/part.lua:313: in function 'remove'
belt-balancer-2/control.lua:174: in function <belt-balancer-2/control.lua:163>
5929.695 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Belt Balancer 2 (2.0.6) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event belt-balancer-2::on_entity_died (ID 4)
belt-balancer-2/objects/balancer.lua:280: Arguments count error for 'spill_item_stack': Expected 1 argument but 7 were given
stack traceback:
[C]: in function 'spill_item_stack'
belt-balancer-2/objects/balancer.lua:280: in function 'empty_buffer'
belt-balancer-2/objects/balancer.lua:258: in function 'check_track'
belt-balancer-2/objects/part.lua:313: in function 'remove'
belt-balancer-2/control.lua:174: in function <belt-balancer-2/control.lua:163>"
5929.695 Info ServerMultiplayerManager.cpp:805: updateTick(3389380) changing state from(InGame) to(Failed)
5929.695 Info SteamContext.cpp:530: Leaving lobby...

This is fixed now