I always test the recent updates on my wife's Macbook Air, where I have Logic Pro 9, and often have this problem. I don't know how this solves the problem but I always go to the Plugins/Components folder and click on one of the Nebula plugins, after which I restart Logic and Nebula is back in the plugins list. I don't have a license on that computer - I just test the upgrade for any problems. However, I have never had this problem on my proper Nebula system - upgrades have all gone smoothly so far. Just suggesting you try this.