I am getting the following LUA error while hovering over the minimap icon in 9.1.5, but it mentions UndermineJournal addon as well so I'm not quite sure where the issue resides:

Message: ...dOns\SavedInstances\Libs\LibQTip-1.0\LibQTip-1.0.lua:543: attempt to call method 'GetBackdrop' (a nil value) Time: Wed Nov 3 00:36:31 2021 Count: 1 Stack: ...dOns\SavedInstances\Libs\LibQTip-1.0\LibQTip-1.0.lua:543: attempt to call method 'GetBackdrop' (a nil value) [string "@Interface\AddOns\SavedInstances\Libs\LibQTip-1.0\LibQTip-1.0.lua"]:543: in function <...dOns\SavedInstances\Libs\LibQTip-1.0\LibQTip-1.0.lua:539> [string "@Interface\AddOns\SavedInstances\Libs\LibQTip-1.0\LibQTip-1.0.lua"]:132: in function Acquire'
[string "@interface\AddOns\SavedInstances\Core\Core.lua"]:3454: in function ShowTooltip' [string "@Interface\AddOns\SavedInstances\Core\Core.lua"]:2555: in function OnEnter'
[string "@interface\AddOns\DBM-Core\Libs\LibDBIcon-1.0\LibDBIcon-1.0.lua"]:77: in function <...AddOns\DBM-Core\Libs\LibDBIcon-1.0\LibDBIcon-1.0.lua:60>

Locals: tooltip = {
0 =
scrollChild = {
}
scrollFrame = {
}
}
key = "SavedInstancesTooltip"
(*temporary) = nil
(*temporary) = GameTooltip {
0 =
SetQuestLogRewardSpell = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
textLeft1Font = "GameTooltipHeaderText"
ItemTooltip = {
}
SetCurrencyToken = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
SetGuildBankItem = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
updateTooltipTimer = 0.061000
SetQuestRewardSpell = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
textRight2Font = "GameTooltipText"
comparing = false
TopOverlay = {
}
SetPetAction = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
SetLootItem = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
SetQuestItem = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
hasMoney = 1
waitingForData = false
SetTrainerService = defined @interface\AddOns\TheUndermineJournal\libs\LibExtraTip\LibExtraTip.lua:419
numMoneyFrames = 1
shoppingTooltips =

Some info here: https://us.forums.blizzard.com/en/wow/t/backdroptemplate-issues/1125674/8

Haven't found anything official, so far...

Got the same issue, tryde to make some changes but did not work

made a quick fix that worked for me, untill it fixed by the author,
in file :\SavedInstances\Libs\LibQTip-1.0\LibQTip-1.0.lua:543

Background color is a bit off thoe, at least no Lua error

-- local backdrop = GameTooltip:GetBackdrop()

-- tooltip:SetBackdrop(backdrop)
local GenericBackdrop = {
bgFile = "Interface\Tooltips\UI-Tooltip-Background"
}
tooltip:SetBackdrop(GenericBackdrop)

The problem is in LibQTip-1.0 so we'll have to wait until someone updates it or switch to a fork

After tooltip:SetBackdrop(GenericBackdrop) add

tooltip:SetBackdropColor(0.06, 0.06, 0.06, 1.0)
tooltip:SetBackdropBorderColor(0.15, 0.15, 0.15, 1.0)

with your fav colors

Some more details here: https://www.curseforge.com/wow/addons/libqtip-1-0/issues/49

following what others posted here worked for me.
Line 543 - comment out or remove

local backdrop = GameTooltip:GetBackdrop()

tooltip:SetBackdrop(backdrop

replace with

local GenericBackdrop = {
bgFile = "Interface\Tooltips\UI-Tooltip-Background"
}
tooltip:SetBackdrop(GenericBackdrop)
tooltip:SetBackdropColor(0.06, 0.06, 0.06, 1.0)
tooltip:SetBackdropBorderColor(0.15, 0.15, 0.15, 1.0)

Visual that may help others

Working!

As @Duckwhale referenced, I downloaded https://www.curseforge.com/wow/addons/libqtip-1-0 and it resolved the issue for me without having to edit the LUA file.

LibQTip was updated and we will push an update with the new version soon.