Another weirdness is that if you have given your P's Plugins a custom icon, then although it might appear to have FULL permissions, Windows can have reduced your permissions slightly, this can be sufficient to limit your ability to write/manage files in the folder...
Also, simply being an Admin or having SketchUp 'run as admin' is not sufficient IF Windows has limited your permissions [which is the default for new folders formed in the Programs Files tree]
Your new FULL security permissions for the Plugins folder ought to filter down to apply to its contents too, as these should be set by default to take their permissions from their 'container'. However, it is worth checking this has happened too...