Having similar issue. Oauth was working fine and then starting yesterday it seems to break. When trying to activate now, the prompt attempts to popup but instead leaves a dark tint across the whole page and a white ‘X’ with nothing else. After getting out of the screen the activation button can’t be hit anymore until the page is refreshed and then the same thing happens all over again.
Also, I thought I remember having an Oauth application automatically registered for me where I was able to change my oauth path and it was working fine. And now I can’t seem to find it again as if it got deleted somehow. I also tried default URI again no luck.