If you are running a downgraded Skyrim (1.5.97), you cannot use the latest USSEP from Nexus Mods. You need the archived "1597" compatible version.
Here is the crucial reality check:
mod. This allows your 1.5.97 game to read newer plugin headers used by current USSEP versions. Updated Master Files: Update.esm Dawnguard.esm HearthFires.esm Dragonborn.esm from the latest version of Skyrim, but keep your 1.5.97 SkyrimSE.exe Important Notes skyrim se unofficial patch 1597 download new