Last night there was a bug in Android Market server that caused "Download Error" for people who downloaded version 13.4 of NubiNews. In most cases, if you just download the latest version (13.5 of this writing), you will be able to upgrade successfully.
However, in the worst case (it actually happened to me during testing), your Android Market app will be stuck in a state where it keeps saying "failed to download" NubiNews.
The only solution is go to Settings -> Applications -> Manage Applications. Choose "Android Market". The Choose "Clear Data" and "Clear Cache".
This is a bug in the Android Market server. I am sad to say I had nothing to do with this and can't do anything about it.
If you have problems, please write to firstname.lastname@example.org.