I upgraded a row yellow belt and underground belt when the error occurred.

8219.284 Info AppManager.cpp:304: Saving finished
8641.854 Error MainLoop.cpp:1456: Exception at tick 4345370: The mod Updated Construction Drones - Forked (0.1.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Construction_Drones_Forked::on_ai_command_completed (ID 135)
Unknown item name: entity-ghost
stack traceback:
    [C]: in function 'insert'
    __Construction_Drones_Forked__/script/inventory_manager.lua:100: in function 'take_entity_stack'
    __Construction_Drones_Forked__/script/command_processor.lua:649: in function <__Construction_Drones_Forked__/script/command_processor.lua:600>
    (...tail calls...)
    __core__/lualib/event_handler.lua:72: in function <__core__/lualib/event_handler.lua:70>
8641.854 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Updated Construction Drones - Forked (0.1.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Construction_Drones_Forked::on_ai_command_completed (ID 135)
Unknown item name: entity-ghost
stack traceback:
    [C]: in function 'insert'
    __Construction_Drones_Forked__/script/inventory_manager.lua:100: in function 'take_entity_stack'
    __Construction_Drones_Forked__/script/command_processor.lua:649: in function <__Construction_Drones_Forked__/script/command_processor.lua:600>
    (...tail calls...)
    __core__/lualib/event_handler.lua:72: in function <__core__/lualib/event_handler.lua:70>"
8641.854 Info ServerMultiplayerManager.cpp:808: updateTick(4345370) changing state from(InGame) to(Failed)

I was unable to reproduce the error after loading and doing the same thing again.
I'm playing with a few mods but it was the first time in ~20 h.

Not sure what happened here. The error implies that it tried to take an entity ghost out of the ether to place for a neighbor (instead of whatever you were trying to place), specifically, an underground belt of some sort.
Based on the events in this log, I'm not seeing the error when trying to duplicate it locally either.
If you are able to recreate it, let me know

I was able to reproduce it once in several attempts. The log seems identical to the above.
I've uploaded a screenshot to better explain what I do. The lower part (white) is already built and working. I place the upper part (blue) via blueprint and while the bots are building, I drag an upgrade-planner down the one-tile-wide lane (red arrow). Initially I did that last part because I ran out of yellow underground belt to finish it.