If MainStage/Logic is trying to load a third party plugin you have deleted but is getting stuck, check out this article from Apple.
You'll want to:
- Navigate to Finder, and select Go > Go to Folder, and head to this location:
- Look for any files that might belong to the deleted plugin - delete if there are any.
Does the problem still persist? You might need to force rescan AU plugins by deleting the Audio Units Cache. Navigate to “~/Library/Caches”, delete “AudioUniteCache” folder. Warning, this might also require Mainstage to rescan your plugins.