It seems there's a major issue with Imgur and the Upload extension as many users seem affected.
Unfortunately I'm not using it myself and I don't think any FriendsOfFlarum member is running the Imgur adapter on their own website.
I've done a quick search after the first report and again now but I'm not finding any other report of people with issues (I mean non-Flarum users).
I've also reviewed the Imgur API documentation but it doesn't appear anything changed recently.
The problem is that the error appears to be just "400" with no indication of what the problem is.
You should check whether your Imgur token is still valid in Imgur API settings. Maybe they revoked many tokens or something like that.
If you could contact Imgur support and let them know you have an issue, maybe they'll be able to give some hints at what it could be.