Bugfix v3g is a patch, therefore it must be copied into the game folder of the vanilla game before any mods are applied.
Either reset all mods in game, or if that isn't working, then cut and paste the contents(minus the version file) of the "backup" folder into the game's "files" folder. Windows will give a notification allowing you to replace and merge everything. You should delete the "backup" folder if it exists. To apply the patch, do basically the same thing, copy the contents(minus Changes.txt) of the patch into the game's "files" folder, replace and merge.
The patch alters the way the mod system works, so Leo's mod won't put stuff in the right places and critical stuff will be missing.