The prefs window opens as usual for Zotero's own preferences, it's just that the BBT code in my overlay for the prefs window does not run, so my prefs UI never gets initialized. It does not seem to fix itself.
It could well be that the message is benign, but it's the only error that showed up in these users' logs, and the code in my overlay simply does not run at all -- I had a Zotero.debug as the first call, and that didn't show up.
Unfortunately it's been two weeks since the last message from these 3 users came in, and I can't reproduce it myself. One promised to test end of last week, but I've seen nothing yet.