During earlier beat-testing I understand that although your PC's Properties > Security might report FULL for you... when you add a custom icon to a folder [like Plugins] it can downgrade your permissions just a tiny bit - it still says FULL... 'but not quite' - and then some installations attempted from tools like the RBZ one in Preferences > Extensions could fall over, as you are seen as having restricted permissions, no matter how slightly curtailed ! So if you have added custom icons to your SketchUp folders please watch out for any potential unexpected and inexplicable installation issue... It might never happen, but if it does this is a possible cause...