I think this is the same bug being discussed here: Issue with Audio Unit on 10.11 - #22 by jaydee
It affects some plugins on the users’ systems, but not all of them. Unfortunately, my plugins using Juce are in the group that no longer works.
Juce team: any idea what is going on here? I’d love to have a solution, versus “yeah, Apple broke some plugins that happen to use Juce, sorry.”