I suspect another addon is reducing Questie's frame level to zero, so the code in the OnShow handler (QuestieFramePool.lua line 314) tries to set the FrameLevel to -1.

self.glow:SetFrameLevel(self:GetFrameLevel() - 1)

I changed it to this:

local oldFrameLevel = self:GetFrameLevel()
if oldFrameLevel > 0 then self.glow:SetFrameLevel(oldFrameLevel - 1) end

Are the frames still sorting in a correct manner?

I don't know which frames are involved here, and what's being sorted. Is this to keep the tracker window floating above or below other windows?

Added the safety code to atleast keep it from erroring out, more research is probably needed to figure out the root of the problem.