Many if not all modlists touch them in some way. P.P.P.P.S Thanks to user Floydfire the problem with heavy modlists and as a result not working quests lies in cleaned update.esm and/or any modified original. But do it at your own risk, as always make backups before proceeding!
#STEAM API DLL MISSING CREATION KIT PATCH#
But if you use some minimal mods like SkyUI, Engine Fixes Unofficial Patch then it is probably going to be ok. To conclude: if you have a huge modlist with tons of graphics, new quests, gameplay mechanics mods it is best to not add AE content. Apparently some mods break certain elements in the updated BSAs and ESLs which is to be expected. So I did a bit of testing again and can confirm that "The Cause" quest also works with 1.5.97 executables, the problem appeared when I decided to use minimal modlist from Wabbajack "Legends of The Frost" for 1.5.97 and the courier didn't show up. P.P.S Ok, I managed to get SSE Engine Fixes work with this "partial" downgrade Just checked fishing and I could grab a rod and catch a fish and then some quest started. Of course it needs more testing, maybe something is broken but at least now you can use essentials without waiting - SSE Engine Fixes, Unofficial Patch and so on. I could buy horse armor and had the prompt about whether turning survival mode on or off. Make sure to change only these 3 files, all of the files inside Data folder should be from AE version! What I did is replaced skyrim.exe, skyrimlauncher.exe and binkw64.dll with the ones from 1.5.97 and the game loaded without hastle. As someone has mentioned, apparently you can downgrade your anniversary edition to the latest special edition and still have the new content.