Latest versions of both addons on 11/03 (will edit version numbers later)
https://i.imgur.com/wRXUtXr.png
https://i.imgur.com/9EwtkDN.png

I don't know who is responsible for fixing this, created an issue on both repos
Rottenbeer/ItemRack#148

Is it possible that the order of loading might change in the future? If so, you might add code to handle the case where IR is already loaded.

I posted in the AdiBags-ItemRackSets issues already, but I'll put this here in case it helps anyone:
Seems that ItemRack:RegisterExternalEventListener gets loaded later so ItemRack needs to finish loading before it can be used.
Not sure if this is the ideal fix, but I was able to get mine working again by changing a little bit in the Adibags-ItemRackSets.lua file -
On line 35/36 I changed:

ItemRack:RegisterExternalEventListener("ITEMRACK_SET_SAVED", itemRackUpdated)
ItemRack:RegisterExternalEventListener("ITEMRACK_SET_DELETED", itemRackUpdated)

to this:

local IREvents = CreateFrame("Frame", nil)
IREvents:RegisterEvent("ADDON_LOADED")
IREvents:SetScript("OnEvent", function(self, event, addon)
    if (addon == "ItemRack") then
        ItemRack:RegisterExternalEventListener("ITEMRACK_SET_SAVED", itemRackUpdated)
        ItemRack:RegisterExternalEventListener("ITEMRACK_SET_DELETED", itemRackUpdated)
    end
end)

AdiBags-ItemRackSets is a third-party addon, so this issue definitely belongs there. However, ItemRack's addon author has already implemented a workaround.