FortiAP 14C firmware upgrade to 5.4.1 from 5.2.4 kills admin console web interface?

So we've got a bunch of these little FortiAP 14C's for our remote users, and really, what a neat, slick solution.  We really like them, but we also really like being secure and up-to-date.  We have been lax on that lately, and most of our 14C's are running an older version of the FortiAP software.

Wanting to rectify that, I grabbed one of the ones that we seem not to be using, and started playing with it.  It had version 5.0.9 build 0086 on it, and per this guide, I needed to proceed according to the following upgrade path:

5.0.9 > 5.2.3 > 5.2.4 > 5.4.1 > 5.4.2 > 5.6.0

All was well, until I got to version 5.4.1 build 0339.  After upgrading to that, and then to all subsequent firmware revisions, I could no longer access the admin webpage of the FortiAP device I was working on.