Thanks a lot for the report! Sorry for the late reply, this somehow slipped my attention.
Hm ... technically, I'm checking if the app is active before uploading anything. So I don't see how this can happen. Nevertheless, as you said: it's likely due to the new AssetManager.
Do you happen to have a complete callstack of one or more of those errors?
(For reference, error 3768 means: The Stage3D API may not be used during background execution.)