All tracks are (or at least WERE audio...read further.) I had one composite MIDI track for drums, which I froze.
Disabling Fast Bounce made no difference.
Just for ha-has I unfroze the drums, at which point the following message box popped up:
"Addictive Drums 2
Wrong binary location
You are running a binary that is not the same as the one updated by the XLN Online Installer.
Please check your path settings by clicking the Advanced button in the XLN Online Installer and make sure that it matches with the plugin path setup in your host."
Which I think is a fancy way of telling me the AD2 plugin is not where XLN thinks it should be. So I moved it to the proper folder and re-scanned my plugins, and now I no longer get that message. However, now there's some other AD2 error message (for which I will probably have to contact XLN, because I have no idea what it means), and when I freeze the drums it produces no drum audio on the audio track associated with the drum MIDI data.
I have no idea as to whether or not this problem is related to the export problem.