As of WoW Classic patch 3.4.1 (Retail Patch 10.0.2), blizzard updated some tooltip related APIs.

https://wowpedia.fandom.com/wiki/Patch_3.4.1/API_changes

Since then, on login, both GetContainerNumSlots and GetContainerItemID throw errors.

Fix:

main.lua
line 1382
change:
for j=1, GetContainerNumSlots(i) do
local _, qty, _, _, _, _, _, _, _, itemId = getCached(cache, GetContainerNumSlots, i, j);
to:
for j=1, C_Container.GetContainerNumSlots(i) do
local _, qty, _, _, _, _, _, _, _, itemId = getCached(cache, C_Container.GetContainerNumSlots, i, j);

Inventory-EditBox.lua
line 24
change:
for j=1,GetContainerNumSlots(i) do
local itemId = GetContainerItemID(i, j)
to:
for j=1,C_Container.GetContainerNumSlots(i) do
local itemId = C_Container.GetContainerItemID(i, j)