I've been having the same issue since last patch. However for some reason it only occurs after like 1h of playing or in 32-bit mode.
Pretty sure its not an AtlasLoot issue as I see this message from time to time on my screen quoting other addons, e.g. Shadowed Unit Frames. No idea why it happens but doesn't happen often enough for it to be an issue for me personally.
Yeah, Blizzard messed up again. It'll be fixed once the next patch hits live, until then you can get an addon that'll suppress the error.