The mod Inserter Fuel Leech (1.0.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event InserterFuelLeech::on_tick (ID 0)
LuaItemPrototype doesn't contain key fuel_category.
stack traceback:
[C]: in function 'index'
__InserterFuelLeech/inserterTools.lua:39: in function 'getInserterMovableCountOfItemStack'
InserterFuelLeech/inserterLeechFuel.lua:67: in function <InserterFuelLeech/inserterLeechFuel.lua:56>
(...tail calls...)
InserterFuelLeech/inserterUpdate.lua:91: in function 'handleOwnFuelInventoryEmpty'
InserterFuelLeech/inserterUpdate.lua:229: in function 'updateInserter'
InserterFuelLeech/inserterRepo.lua:216: in function 'updateInserters'
InserterFuelLeech/control.lua:66: in function <InserterFuelLeech/control.lua:63>

This confirms Farlex's report. So the 2.1.20 update did indeed change fuel_category into fuel_categories. Good to know.

I updated the mod.

Thank you ^^