I've noticed this particular bug on the most recent SDK (3.6.1). Any attempt to upload an avatar or world (using VRCWorld prefab), will fail with [Attempted to load the data for a world we do not own, clearing blueprint ID]. This only happens if the object you are uploading is a prefab. This is most inconvenient with avatars since most avatar bases are setup as prefabs. The simple solution is to right click and unpack the prefab in the scene, but it is annoying nonetheless.
And no, there was no previously attached content id.