Could simply be an effect using up too much RAM or some of your 3D stuff like large shadow maps. Even damaged source footage can cause this. Impossible to tell. You really can only get to the bottom of such issues by closely monitoriung your renders. It may help to split your render in separate segments in the first place. That way AE wil lfree up as much RAM as possible before starting a new job on the queue...
It said something about "shadow fragments", so I'm tempted to think you are spot-on about the shadow maps.
I didn't use many effects, but all the images I used were very high-quality, so I'm guessing that's the problem.
This is a music video, so if I render separately, how will I be able to piece together the entire thing? By re-importing the renders and piecing them together and rendering that? Is that the best way.
Okay so after reading more, I think it's because the images I scanned in are too large and scaled down in AE. For my next project, I'll know, but for this project, is there a way to know how to reduce them appropriately and substitute them so that I won't have to do any new keyframes or something? A way that I can just edit the files in Photoshop and have them update to be lower res in AE without affecting their sizes?