UAL.com tells me that it is a known bug -- generally when you have a car and/or hotel on the end of your itinerary and you've booked through a third party travel agency.

What you can try to do is book the flight, get it ticketed, then do the upgrade. After you complete the upgrade, open the itinerary up again and book the hotel and car (if you have an online booking agent that allows that).

BTW, it took several calls to UAL.com to find out that this was the issue. Had to have the company travel agency call their direct support, who then asked a supervisor -- only then did we find out that it was a known bug.

Only affects the online upgrade tool.

The funny thing is that it is something recent -- say in the last three months or so. It worked earlier in the year without a problem, so somebody changed some code and didn't test it properly.