Very quick post here.
So I recently retargeted a WP8 app to WP8.1 (Silverlight) and tried adding a background task, but kept getting an UnauthorizedAccessException when attempting to register the task.
As it turns out it seems retargeting an app might be slightly buggy here and there, and I just couldn’t get it to work. On the other hand a new 8.1 project with all the code/content of the old project dragged into it worked just fine.
(Well, that was half a day wasted.)