You might want to look at a posting called "Avoiding the "After Effects error: could not create image buffer""
on Jonas Hummelstrand's blog - it explains how to avoid that error. See
http://generalspecialist.com/2006/11/avoiding-after-effects-error-could-not...
Unfortunately one of the recommendations if you are running into that specific scenario is disabling Nucleo Pro (or Adobe's multiprocessing) to allow a single process as much memory as it can get. There are also settings that can be altered to help.
Although you may not have a layer or any one footage item that is that resolution there may be an intermediate step that AE takes to render that frame that requires a large buffer.
Chris
GridIron Software - Verification and Customer Support
Check our Frequently Asked Questions (http://www.gridironsoftware.com/pg/Nucleo_FAQ)