Actually, the runtime for SKSE64 2.0.4 is 1.5.3 Skyrim Updated just recently to 1.5.16 so 2.0.4 is outdated for the current version of Skyrim. You'll have to either 1) Wait until the SKSE team manages time to update SKSE64, or 2) Follow instructions posted previous to your post on how to download previous versions of Skyrim SE. Also, Don't think this will be the only time this will happen if you have automatic updates set to settings to keep the game automatically updated. It's probably best to set it so that it only updates when launched through the Steam app, so that it doesn't update when using SKSE launcher. This is a mistake that I myself am fixing.