I'll look into it. I think I know the issue. I also use RBM and Xloot together. So I would try updating your xloot :p
My XLoot is the latest version from wowace. Maybe some weird interaction with a another addon or something? I can try doing some more testing. All I know is if I enable Xloot with RBM it messes up PitBull and FuBar bad but if I disable it, it works fine.
Okay I narrowed it down to a combination of addons if I have just the following addons enabled I have the error, if I disable any of these I do not get the error.
The error is:
Interface\Addons\BigWigs\Libs\AceLibrary\AceLibrary.lua:218:cannot change a protected meta table
(note that in testing I've seen variations on the exact addon that throws the error but it is always AceLibrary.lua:218)
The addons are:
Beancounter (with it's dependency Stubby)
RBM (just core)
So I can't blame any one of these because I don't know what the interaction is that is causing the problem (I don't know LUA well enough to go further at this time.)
For now I'm just going to disable Beancounter since I don't really use it much and run that way.
Now to look at RBM in more detail... well tomorrow maybe...