Skyrim+skse+16640+2021 -

The "Stealth Update" Bug. Even if you have SkyrimSE.exe set to 1.6.640, Steam might have downloaded the 1.6.1170 executable into a temporary folder. Or, a mod manager changed your root directory.

Fix:

SKSE 1.6.640 expects the executable name to be exactly SkyrimSE.exe. If you renamed it (e.g., to SkyrimSE_1640.exe for mod profiles), SKSE will fail. Rename it back.

When people search for "skyrim+skse+16640+2021," they aren't just looking for the launcher. They want to know which mods from that era are compatible.