I have been constantly getting this Lua error for a long time now, and i've finally decided to try and fix it, i've run AceUpdater countless times, and redownloaded+installed the ace2 modules from Curse, but nothing seems to fix it.
I get this error message every time I finish a loading screen(logging in, entering/leaving an instance/arena, etc) and it's very annoying. I tried some basic troubleshooting and created a toon, turned off all mods except the Lib mods and entered the realm, and the error still popped up.
This seems rather self-explanatory...you're somehow missing AceEvent-2.0.
My guess would be that one of your mods is using AceComm-2.0 without properly embedding AceEvent-2.0. A cheap-fix would be install Ace2 stand-alone...but you should really find out which mod your using that is causing the issue, and contact the author.