Hello @valeth, we'll be tracking game compatibility in one issue report per unofficially supported game so I've gone ahead and transferred this to https://github.com/ValveSoftware/Proton/issues/4#issuecomment-415512908.
@kisak-valve I think this was merged into the wrong issue; it looks like (and is confirmed by my experience that) this is a problem with the non-SE version of Skyrim.
When trying to load mods in Skyrim an error message appears on the launcher with the message
Failed to extract plugin file from ckm!.This does not prevent the game from being launched, it just skips initializing any third party mods (the high resolution texture packs seem to work, they are at least listed in the
Data Filesmenu).System Info