This happens when:
- the user tries to follow a story level from a review
- the game receives a story level from the presence server
- the user selects a story level in a custom category in LBP3
- probably in some other obscure cases I haven't found yet
This can be worked around by returning these levels as user levels in these cases (slot type user + real ID, this approach would greatly benefit from #196 being implemented); maybe actually setting a publisher username for story levels could also be helpful to catch unknown cases, although afaik having a publisher username on a developer level will make the game not spam the server, but fail to load the user anyways in such cases.