My friend did this and reported it to me. The title includes what I know :P.

Error while running event Kux-SlimInserters::on_robot_built_entity (ID 14)
Kux-SlimInserters/events/on_built_entity.lua:112: attempt to concatenate local 'new_prefix' (a nil value)
stack traceback:
Kux-SlimInserters/events/on_built_entity.lua:112: in function 'on_built_slim_inserter'
Kux-SlimInserters/events/on_built_entity.lua:58: in function <Kux-SlimInserters/events/on_built_entity.lua:43>
13897.786 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Kuxynator's Slim-Inserters (3.0.17) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Kux-SlimInserters::on_robot_built_entity (ID 14)
Kux-SlimInserters/events/on_built_entity.lua:112: attempt to concatenate local 'new_prefix' (a nil value)
stack traceback:
Kux-SlimInserters/events/on_built_entity.lua:112: in function 'on_built_slim_inserter'
Kux-SlimInserters/events/on_built_entity.lua:58: in function <Kux-SlimInserters/events/on_built_entity.lua:43>"

Double slim inserter on the double "slim" inserter.
Missing the word on the title.

I almost thought so, there are no other double inserter in factorio ;-)
sound like bluprint build (on_robot_built_entity )
i have tried everything possible but i could not reproduce the error.
there are some other strange things if dual inserters mixed up with double inserters, I still need to look at, but no crash.
I would need to know the exact action and preconditions to be able to reproduce this.

Yep. I have no idea how to reproduce it too.
I will report again if I or my friend did it again.
I also think if any possible the reason comes from the Team VS scenario.

https://drive.google.com/drive/folders/1CJNy5LukOYmiRMTr8RCO0TCyjb_aLsth?usp=sharing
I got the video and the save from my friend :D
It seems like the robot suddenly goes to destroy (or rebuild?) the slim inserter, but no one orders it to do so when my friend removes one of the slim inserters. But somehow the slim inserter's texture seems like get bugged and showing only one side of the double slim inserter. (Just so weird)

My friend thought that it could be the case that the robot tends to grab the slim inserter from the bottom one to build the top one, which causes the crash.

Hope it can help to figure out what's going on.

Bug Log:
68.933 Error MainLoop.cpp:1429: Exception at tick 4755237: The mod Kuxynator's Slim-Inserters (3.0.17) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Kux-SlimInserters::on_robot_built_entity (ID 14)
Kux-SlimInserters/events/on_built_entity.lua:112: attempt to concatenate local 'new_prefix' (a nil value)
stack traceback:
Kux-SlimInserters/events/on_built_entity.lua:112: in function 'on_built_slim_inserter'
Kux-SlimInserters/events/on_built_entity.lua:58: in function <Kux-SlimInserters/events/on_built_entity.lua:43>
68.933 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Kuxynator's Slim-Inserters (3.0.17) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Kux-SlimInserters::on_robot_built_entity (ID 14)
Kux-SlimInserters/events/on_built_entity.lua:112: attempt to concatenate local 'new_prefix' (a nil value)
stack traceback:
Kux-SlimInserters/events/on_built_entity.lua:112: in function 'on_built_slim_inserter'
Kux-SlimInserters/events/on_built_entity.lua:58: in function <Kux-SlimInserters/events/on_built_entity.lua:43>"