This happened to me once in the past on C2 and a big project too. In my case it came from my way of including event sheets. Maybe trying to look on the side of your includes and the moment they are called (and indeed on the variables side as well). You can also create a global event sheet to include all your includes, which will be called by your layouts. If it can help you debug, at least it helped me to see more clearly. Good luck.
IT WORKS NOW! For absolutely NO REASON AT ALL, it just starts working out of nowhere. I don't know why or how. I went to go fix some other (completely unrelated) issues that I had and came back to see if I could try again, and LO AND FREAKIN BEHOLD, it works now.
Just as mysterious as it stopped working. It's working again. It's kind of bittersweet in a way. I'm ELATED that it's working again, but I'm still kind of upset that I don't know what happened.