Autoupdate fails with Error #16824

I just deployed a new version of my application, and autoupdates are failing with the dreaded Error #16824.

However, I have not upgraded my SDK in between releases, nor have I altered anything in my update.xml other than the version numbers. And the build and deploy processes is automated, so that should not have changed either. What could be going on here?

Old update.xml:

<?xml version='1.0' encoding='UTF-8'?> 
 <update xmlns='http://ns.adobe.com/air/framework/update/description/1.0'> 
   <version>2.1.22</version> 
   <url>http://ramp.leancoder.com/client/downloads/ramp.air</url> 
</update>

New update.xml:

<?xml version='1.0' encoding='UTF-8'?> 
 <update xmlns='http://ns.adobe.com/air/framework/update/description/1.0'> 
   <version>2.1.26</version> 
   <url>http://ramp.leancoder.com/client/downloads/ramp.air</url> 
</update>

Both versions were built with:

$bin/adt -version
adt version "2.0.2.12610"