I had the same problem this morning (I noticed upon arriving to work that the token used has been flagged unauthorized).
I had the same error code as you when I tried to issue a new token.
We also have an app, but the instagram test user has been reset. Upon resetting it, I've noticed meta has updated the profile management on instagram and switched it to a "meta profile", maybe this update has disconnected the previously linked applications to the instagram user.
So I had to link the app to the profile again, obtain a short-lived token, exchange it for a long-lived one and hurray, got a new valid token. Unfortunately the lib I was using has not been updated for 2 years and maybe, if changes were made to instagram API, it will not work again.
I haven't found much online about this new feature but a post here on stackoverflow by Google Cloud :
Facebook.com/down & outage today
Indeed "stack is not a reporting service" but I've also noticed 2 or 3 threads about the meta / fb / insta api since the last 10 hours.
... Oh and about threads: instagram is launching threads (a twitter's competitor) on july 6th. That could also explain why they have to expediate updates to get the new platform ready.