Just am FYI, I was having the above critical error whenever I tried to open WoW, turns out the interface addons folder does not like to have folders or executables in there which are not addons. I had dragged and dropped a folder there by mistake (I have a shortcut to that folder on my desktop) and was causing the error, guess it was trying to load what ever file/program it was as an addon which it was unable to.
Issue is fixed but this was not specified as a potential solution / check on the support page (though deleting the whole interface folder would)