Since the launch of 8.1 and yesterday's update to ATT, chat is being mass spammed by LUA errors every 10 seconds, more or less non stop.

"Interface\AddOns\AllTheThings.lua:9895: attempt to index local 'text_to_mapID' (a nil value)"

ATT also doesn't appear to be active or accessible in game. Any idea what's going on?

This was fixed in versions 1.7.8a-d.

Make sure that your client is properly using the correct version. Blizzard changed the addon paths for it's addon. So when it migrated it copied all your old addons over. Twitch tries to correct this, but on some clients it is defaulting to the old directory so you update a non existant version. You will want to make sure you chose the option that says

_retail_

As indicated in the bottom "World of Warcraft"