After a lot of fruitless Google work (911 is a noisy search term), even checking the unofficial Every Sprint 3g Error Code page, I settled in on trial and error.

It turns out that this is a kind of permissions problem, caused by the “Download Domain” of the new version not matching the old one.

When an application is downloaded the device stores the hostname part of the URL, any updates to the application must come from the same hostname. So if you downloaded it from www.foo.com the first time, you can’t update it from m.foo.com.

On the v9m (RAZR 2) you can find out what the current “Download Domain” is for a specific application by going to the “Applications” folder under “My Content” and bringing up the “Options” → “Properties” menu for it. The “Download Domain” will be listed at the bottom of the scrollable page.