I struggled for hours with AE cc2017 trying to render a sequence at 3840 X 2160. I have a mid-2012 MacPro with 48GB ram and my GPU is an NVIDEA Quadro. I could render the sequence out at 1920X1080 just fine. I could render other sequences out at 3840 X 2160. It was just this one sequence that had a lot of large still images in it and nested sequences for which I’d created 1/2 res proxies.
I thought, silly me, that if I just selected “use all proxies” or “use current settings” in the render settings tab of the AE render cue that I would be taking the strain off my machine for rendering all those complex effects in the nested sequences. I created lots of half rez proxies (which I could live with) and I made sure they were all enabled in the project window, but there just was no getting the sequence to render to 3840.
What finally solved my problem was finding the proxy files I’d created (right click on the proxied comp in the project window and select “reveal in finder”) and import the proxies and replace the nested comps with these proxies physically in the final comp. A sort of “ignoramus’ prerender” workaround, but it worked and I was finally able to render out at my desired resolution.
I suppose PRERENDERING that which you were using PROXIES for is the answer. Proxies seemed to muck up my render cue. Once I did that I was also able to render using Adobe Media Encoder which is much prefered.