I found the answer to my problem, by continued searching for web postings related to failed WiFi associations. I'm a bit embarrassed to admit...
The problem was that I'd enabled MAC address based connection limitation on my WiFi Access Point. I did that back when the 802.11i key-reuse security bug was originally announced. I forgot that I had done so. Ooops.
So, it seems that MAC based association rejection comes back as an "unknown reason".
This message was posted on behalf of Intel Corporation
That can happen to the best of us, specially with features that are often set up once, then never touched again. We're glad to hear you were able to figure out the source of the issue.
Please don't hesitate to contact us again if there's anything else we can help you with.