I know I have a valid appcast, as older versions of the application still work. I believe the problem started occurring around when I became a registered Mac App Developer and installed my certificate in Xcode, however, turning off code signing does not ameliorate the problem. This is the error message:
2013-03-21 19:07:14.485 Mac Linux USB Loader[2312:303] Sparkle Error: An error occurred in retrieving update information. Please try again later.
2013-03-21 19:07:14.487 Mac Linux USB Loader[2312:303] Sparkle Error (continued): The operation couldn’t be completed. Operation not permitted
Sparkle then just displays a generic NSAlert telling me that it failed. I tried signing the framework but that did not fix the problem.
Is there something more specific in ~/Library/Logs/SparkleUpdateLog.log?
I'll check on that.
I looked through the logs and found my problem. I had forgotten to add network capability for the update checker to the entitlements file. My bad!