Starting from iOS 9, Apple will no longer provide a way to trust enterprise developer profiles when they are first used. The user will have to dig through the settings manually in order to use Appeon Workspace with Apple Enterprise certificate. So for iOS9 users, who never installed Appeon Workspace before, please do the following:
1. Install Appeon Workspace.
2. Dismiss the message (Untrusted Enterprise Developer) when you are prompted.
3. Go to Settings > General > Profiles and tap to trust the Appeon certificate.