What kind of 400mm f/2.8? AF-S, AF-I, or is it one of the older MF versions? Teleconverter or not? Are you mounting the lens or the camera on the tripod?
Assuming that you're not using a teleconverter and it's an AF-S lens, I'd be suspicious of the camera/lens contacts at the top of the mount. I've seen isolated cases of the spring-loaded contacts not working correctly.
author, Nikon Field Guide
author, Nikon Flash Guide
author, Complete Guide to the Nikon D1, D1h, & D1x www.bythom.com