The pure Vanilla start (without LAL) fails the moment you pass the Helgen gates, claiming that you need to use a new game for Requiem. I think there's a cell boundary at this place, so it's very likely that this was caused by our recent fix to get rid of the duplicate error messages. This bug makes the Vanilla start unusable, but can be circumvented by using LAL to start the Vanilla start.
Changes from RRO-313 are reverted.
The Vanilla start works as expected without any third-party mods.