Some mods that depend on USSEP also expect the AE version of the Address Library. Solution: Install the "Address Library for SKSE Plugins (All in one)" – version for 1.5.97. There is a specific file on Nexus under "Old Files" for pre-AE.
USSEP acts as a foundation for almost every other mod in your load order. It modifies core game records, meaning it must be loaded immediately after the official game files. Set your load order exactly like this: Skyrim.esm Update.esm Dawnguard.esm HearthFires.esm Dragonborn.esm [All your other mods go here] Unofficial Skyrim Special Edition Patch 1.5 97 Download