Success!
I figured out what caused this problem, and was able to fix it on my AVIC-X910BT using the UI in testmode.
Here's the problem: when you unpack the .ZIP file and copy it to the SD card using a Mac (as I did), the Mac OS creates some additional files in the SD card's filesystem. They're invisible when you look at the card on a Mac, but apparently the firmware update process copies some of them into the device, and they cause the iGo software (the navigation component) to not start up properly.
To fix this, do the following:
- boot up the AVIC in test mode
- find a stylus and calibrate the screen (since you're going to be clicking on a lot of small targets)
- browse to Device/My Flash Disk/APL2/iGo"
- in the view menu, turn on "details" view
- also in the view menu, pick the "folder options" item, turn off all three checkboxes, and click OK (this allows you to see hidden files)
- look through this directory and all its subdirectories for files that start with ._ (period underscore) and delete them (by selecting the file and choosing "Delete" from the File menu)
I deleted a bunch of these from different subdirectories, so I'm not sure exactly which of them was causing the problem. I suspect the critical one was CONTENT/LANG/._Lang_French_Can.zip, since when I tried to run the EZRIDER.EXE program from windows explorer, it complained that it couldn't load languages.
After deleting the files, removing the testmode SD card, and restarting the car, it booted up and everything seems to work. 