I worked with ESRI Germany via ESS channels to identify this very issue today.
In this specific case, the problem is that the file being uploaded to the ArcGIS Online Web Sharing application had an umlaut in the file name, which is not escaped when the file is uploaded. While I'm not sure, it appears that the AGO database replaces accented characters with ASCII characters, but the URL provided to the resource maintains the accented character. Thus there's a mismatch when the resource is requested. The XML error returned in this case is 'The specified key does not exist'. In response to this issue, NIM057246 was logged. See:
[#NIM057246 Data files with regional dialect characters (like umlauts) in the file name are inaccessible via the ArcGIS Online web sharing application. ]