After the update to 9.0.2 I'm getting the following error when I log into the game. Thanks.
1x WQAchievements\WQAchievements-v9.0.1-4release.lua:2158: attempt to perform arithmetic on local 'itemLevel' (a nil value)
[string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:2158: in function CheckItems' [string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:1913: in function Reward'
[string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:447: in function <WQAchievements\WQAchievements.lua:413>

Locals:
self =

This appears to be happening on first login and seems like it is trying to get info before it is able, slotID is null as well. The problem is that prior to line 2158 there is no check to determine if itemLevel is null or not. Simply adding an if statement just prior to the usage seems to resolve it, from line 2157 in WQAchievements.lua:

				local itemLevelEquipped = math.min(itemLevel1 or 1000, itemLevel2 or 1000)
				if itemLevel then
					if itemLevel - itemLevelEquipped >= self.db.profile.options.reward.gear.itemLevelUpgradeMin then
						local item = {itemLink = itemLink, itemLevelUpgrade = itemLevel - itemLevelEquipped}
						self:AddRewardToQuest(questID, "ITEM", item, isEmissary)
					end
				end
			end

			-- Azerite Armor Cache

Thanks for the code changes, but it doesn't look like that resolved the issue as I'm still getting an error when logging in. Same basic itemLevel nil value error, but now affecting different lines of code.

5x WQAchievements\WQAchievements-v9.0.1-4release.lua:2177: attempt to perform arithmetic on local 'itemLevel' (a nil value)
[string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:2177: in function CheckItems' [string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:1913: in function ?'
[string "@datastore\libs\AceTimer-3.0\AceTimer-3.0-17.lua"]:55: in function <DataStore\libs\AceTimer-3.0\AceTimer-3.0.lua:50>

Locals:
self =

Same basic thing, on line 2177 itemLevel is used, on the line just prior to that you should see:

if itemLevel1 then

simply change that to:

if itemLevel1 and itemLevel then

That looks to have fixed the issue, at least there is no error when logging in. Thanks for the help.

After performing the two changes mentioned, I get "\WQAchievements-v9.0.1-4release.lua:2278: attempt to perform arithmetic on local 'itemLevel' (a nil value)", which is the same thing only at a different line.

Why is itemLevel a nil value? It looks like it comes from "itemLevel = GetDetailedItemLevelInfo(itemLink)". Did something change with GetDetailedItemLevelInfo?

As listed by Angryaboutelves. Not sure what changed, but for the last few days no errors were occurring. As of logging in today, I'm getting the same error.

1x WQAchievements\WQAchievements-v9.0.1-4release.lua:2278: attempt to perform arithmetic on local 'itemLevel' (a nil value)
[string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:2278: in function CheckItems' [string "@WQAchievements\WQAchievements-v9.0.1-4release.lua"]:1913: in function ?'
[string "@datastore\libs\AceTimer-3.0\AceTimer-3.0-17.lua"]:55: in function <DataStore\libs\AceTimer-3.0\AceTimer-3.0.lua:50>

Locals:
self =

Before the itemLevel stuff that throws the error I check if itemLink exists.
I noticed that sometimes the itemLink exists, but it's an empty one.
GetDetailedItemLevelInfo returns nothing if the input isn't a proper itemlink, therefore itemLevel is nil and we get the error.

I couldn't test this well, because I couldn't reliably recreate the error.

Let me know if you still get this error.