The 11/11 & subsequent updates relied on more recent versions of .NET than the 1.5.97.0 executable (& supporting files) were compiled with.

AIUI this means that the code is ordered differently, & specific pieces that certain mods (particularly SKSE64) attempt to address directly are in different memory locations from when those mods were created, meaning they need to be updated to use the current addressing if you have a 1.6.xxx.x version of the executable.

Another issue is, again AIUI, some of the changes involved optimizations which mean the executable now does a bunch of stuff faster than it used to, & this has caused crashes for some people because the game now sometimes loads mods before it finishes loading the assets for them, so when it tries to access the assets designated by the mod, they aren't actually loaded into memory, but there is also no relevant error-handling, so the user gets a CTD (also, some of the "mods" where this was a problem were, I believe, some of the larger new Creations).