Just solved the problem.
Turns out the Gmedia Admin Isolation box was enabled by default. (So I did not have to enable it when you recommended it).
So I unchecked that box to give it a try, and now it works.
Of course I did update WP to 4.7 recently, not sure if it is related or not.
No other plugins updated after or just before the last Gmedia update. But i have not created a Gallery in a while, so it could be like that for a while, without being noticed.
I just did what you recommended above, and still the same problem. See Screenshot.
I also tested with Chrome. Same issue.
When I try to enable the IOS app on the Gmedia Admin page I get the message “Something goes wrong…” then nothing.