This is because there is no slag-stone anymore. I should have set the item as hidden instead of deleting it but i didn't thought that removing some stones would cause such a big problem.
Anyways if you disable the mod, load into your save and save the game, reenable the mod and load back in again you should be fine i guess. Just remember how many balancers are beeing removed (make a screenshot or write it down).
Now you can give yourselve the previously removed items back with:
/c game.player.insert{name="balancer-part", count=123}
Similar for the now named slag if you want it back.
To unlock the research again you can use:
/c game.player.force.technologies['belt-balancer-1'].researched=true
This goes up to belt-balancer-3 without bob logistics installed in which case you also want 0, 4 and 5.
Sorry for that.
btw why not use this mod:
https://mods.factorio.com/mod/beltSorter
Its still maintained, does the same and looks better.
