If you downgraded from 1.6.640 to 1.5.97, use SKSE 2.0.20 instead. If you're on newer 1.6.1170+, you need SKSE 2.2.6.
This occurs if you launch the game via Steam. You must always use your mod manager or skse64_loader.exe to boot the game. Essential Companion Mods skse for 1.6.640
To install SKSE for Skyrim 1.6.640, follow these steps: If you downgraded from 1
You can download it from the official site: skse.silverlock.org skse for 1.6.640