[LudiusMaximus]

Apparently PLAYER_LOGIN was just not late enough. It works, when I am using a timer to make sure that I am really the last. Thanks again!!

local folderName = ...
local L = LibStub("AceAddon-3.0"):NewAddon(folderName, "AceTimer-3.0")
local startupFrame = CreateFrame("Frame")
startupFrame:RegisterEvent("PLAYER_LOGIN")
startupFrame:SetScript("OnEvent", function(self, event, ...)
  L:ScheduleTimer("initCode", 5.0)
end)
 
function L:initCode()
  -- Save any previously registered scripts.
  local oldscript=GameTooltip:GetScript("OnTooltipSetItem")
  
  -- Futureproofing, support extra args to pass to previous scripts
  GameTooltip:SetScript("OnTooltipSetItem", function(self, ...)
    
    self:AddLine("I am first!")
 
    if oldscript then return oldscript(self, ...) end
  end);
end