Update:
WoW error window pops everytime a lvl60 or higher mob dies. Also pops whenever a Boss dies. Also pops when turning in quest/rep loot to Thorium Brotherhood. (rep related?)
acual error syntax:
...\Fubar\BabbleLib-Core\BabbleLib-Cor.lua:187:bad argument #1 to 'find'
(sting expected, got nil)