I hope this wasn't posted already.
Using Loaders + Loader Snapping i get this error when trying to upgrade loaders (yellow to red):

Error while running event on_player_selected_area (ID 49)
Error while running event on_built_entity (ID 6)
Expected LuaPlayer, index, or name.
stack traceback:
Loader-Rotation-Fix/control.lua:63: in function 'check_loader'
Loader-Rotation-Fix/control.lua:75: in function <Loader-Rotation-Fix/control.lua:68>
stack traceback:
upgrade-planner/control.lua:757: in function 'player_upgrade'
upgrade-planner/control.lua:628: in function 'on_selected_area'
upgrade-planner/control.lua:599: in function <upgrade-planner/control.lua:598>

Upgrading without Loader Snapping works.

Mods:

Loader Snapping:
https://mods.factorio.com/mods/Articulating/Loader-Rotation-Fix

Loaders:
https://mods.factorio.com/mods/kij336/add-loader