When you're seeing the TXT_KEY values directly, it's nearly always because you have multiple mods (or one mod and the core game) attempting to set the same text key value. You especially see this with new Civilizations, when they might have added a city name that some other mod was also adding. So if mod A is trying to add TXT_KEY_CITY_PASADENA to the "Nerd" civilization, while mod B is trying to add the same key to its "California" civilization, then whichever one is loaded last will fail. (The entire XML update for the second mod will fail, meaning everything in whatever file contained the bad text key.)
But it's also very common after a patch, in general; for instance, before the last big patch, Granaries had no HELP entries (the little blurb you see when you mouse over an icon on the tech tree). But after the patch, they do, so any mod that attempted to do the same would now cause this bug. Many mods simply haven't been maintained recently, and so are now unusable because of bugs like this, and I'm pretty sure Beyond the Future falls into that category, since its thread here hasn't been touched since January. This is the Mod Browser's biggest failing; you just can't tell whether a mod is compatible with the current game.