Not 100% sure when it happens.

10x !Vendorer\vendorfilter.lua:127: attempt to index upvalue 'FilteredMerchantItems' (a nil value)
[string "@!Vendorer\vendorfilter.lua"]:127: in function GetMerchantItemLink' [string "@CorruptionTooltips\Modules\Icons.lua"]:220: in function func'
[string "@ace3\AceTimer-3.0\AceTimer-3.0-17.lua"]:57: in function <Ace3\AceTimer-3.0\AceTimer-3.0.lua:50>

Locals:
index = 10
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index upvalue 'FilteredMerchantItems' (a nil value)"
FilteredMerchantItems = nil
Addon =

{
TryHookUIErrors = defined @!Vendorer\core.lua:1309
MakeTooltipString = defined @!Vendorer\vendorfilter.lua:264
UpdateRepairButtons = defined @!Vendorer\autorepair.lua:195
GetCurrentItemIgnoreList = defined @!Vendorer\core.lua:1015
UpdateMerchantItems = defined @!Vendorer\vendorfilter.lua:564
UpdateVendorerItemLists = defined @!Vendorer\VendorerItemListsFrame.lua:111
ToggleCursorHighlights = defined @!Vendorer\core.lua:446
MERCHANT_UPDATE = defined @!Vendorer\core.lua:469
GetFreeBagSlotsForItem = defined @!Vendorer\VendorerStackSplitFrame.lua:624
DoAutoRepair = defined @!Vendorer\autorepair.lua:93
SetDefaultModulePrototype = defined @ace3\AceAddon-3.0\AceAddon-3.0.lua:409
IsEnabled = defined @ace3\AceAddon-3.0\AceAddon-3.0.lua:451
OpenQuickFiltersMenu = defined @!Vendorer\vendorfilter.lua:724
ScanBindType = defined @!Vendorer\core.lua:796
ScanContainers = defined @!Vendorer\core.lua:723
IsValidClassArmorType = defined @!Vendorer\core.lua:830
UnregisterMessage = defined @ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:153
db = {
}
SetEnabledState = defined @ace3\AceAddon-3.0\AceAddon-3.0.lua:424
FilterText = ""
PatchIgnoreListItems = defined @!Vendorer\core.lua:689
GetMenuData = defined @!Vendorer\settings.lua:94
OnEnable = defined @!Vendorer\core.lua:334
GetItemLinkInfo = defined @!Vendorer\vendorfilter.lua:184
GetQualityString = defined @!Vendorer\vendorfilter.lua:745
GetCurrentExtension = defined @!Vendorer\core.lua:531
OnInitialize = defined @!Vendorer\core.lua:269
UnregisterAllEvents = defined @ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:174
ConfirmSellJunk = defined @!Vendorer\core.lua:1326
IsRedText = defined @!Vendorer\core.lua:809
OpenIgnoredItemsListsFrame = defined @!Vendorer\core.lua:1159
GetName = defined @ace3\AceAddon-3.0\AceAddon-3.0.lua:279
ConvertIgnoreList = defined @!Vendorer\core.lua:665
name = "!Vendorer"
UpdateBuybackInfo = defined @!Vendorer\core.lua:1982
AddMessage = defined @!Vendorer\core.lua:415
Disable = defined @ace3\AceAddon-3.0\AceAddon-3.0.lua:314
UpdateMerchantInfo = defined @!Vendorer\core.lua:1808
RestoreSavedSettings = defined @!Vendorer\core.lua:431
GetItemID = defined @!Vendorer\core.lua:996
HideExtensionPanel = defined @!Vendorer\core.lua:583
GetItemRepairCost = defined @!Vendorer\autorepair.lua:29
GetMinimumSplitSize = defined @!Vendorer\VendorerStackSplitFrame.lua:725
PlayerMoney = 3846479093
GetQualityIndex = defined @!Vendorer\vendorfilter.lua:250
ResetAllFilters = defined @!Vendorer\vendorfilter.lua:583
GetItemTooltipInfo = defined @!Vendorer\core.lua:835
UnregisterEvent = defined @ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:153

Well seems the code was very dumb and occasionally nilled a table variable and then later using it without checking if it was nil.

Attempted a blind (untested) fix, hopefully it works.