I faced next error: while installation skeleton plugin at Jira 7.3.7 or 7.6.1. from Manage add-ons page, an unexpected error occurred message appears and “Installing dialog” doesn’t change its status and doesn’t close . In logs I have found nothing related to this event…
Would it be possible for you to paste the log in here? (Starting a bit before you’re installing it for the first time).
If you refresh the page - is the app installed but disabled or not there at all?