I've had this error probably about 3 times now. It only ever happens in an instance for me and usually after about 3-4 hours.
I don't run ArkInv, I use Baggins.
I do however run cowtip.
I also run Auctioneer.
I'll get the following message "ERROR: Memory allocation error: Block too big." followed by a yes/no window to disable any addons. It takes an ALT-TAB and a WoW "End Task" to close WoW.
Because I am usually mid raid I don't look at error messages. I have noticed though that it seems to occur after a fight while people are looting or if I am mousing about corpses/toons to see if I need to rebuff (I play a pally).
I used embedded libs.
I did recently stop using Xloot and OneBag and went from using Bongos3 to using Bartender4. Other mods I used are all ace driven and they include Cartographer, Omen, FuBar, Prat, Decursive, IceHud, ElkBuffBars, Grid, DrDamage and some I can't remember.
Probably not overly helpful but some might be able to spot a running theme.