Uhh, what makes you think a Unitframes addon has -anything- to do with quest ids?
Just because it's the first addon to load the Ace2 libraries does -not- mean it's the culprit.
Embedded libraries are always responsible for these types of "mistaken identity" Disable all your quest-aware addons, except lightheaded.. then begin enabling them one at a time until the problem returns. voila, you've found the problem addon.
My apologies...I have discovered the culprit. Turns out it is FuBar_QuestsFu.
OK...so I am using the newest version of Lightheaded. When I select a quest, I get this error.
Date: 2008-04-12 11:34:46
Error occured in: Global
Message: ..\AddOns\LightHeaded\LightHeaded.lua line 113:
attempt to compare number with string
Now according to Cladhaire:
This is not an issue with LightHeaded, but with an addon that hasn't been updated to use quest ids instead of name/level pairs. Your stack trace should tell you which addon this is, and it needs to be updated.