You can still extract it from locked posts too: right click the image, copy image location (in Firefox; might be different in other browsers) or open the image in a new tab, and take the URL from there:
https://ip.bitcointalk.org/?u=http%3A%2F%2Fi.imgur.com%2F6vY076t.jpg&t=111&c=xxx
This is the URL you want: http%3A%2F%2Fi.imgur.com%2F6vY076t.jpg
Replace %3A with ":" and %2F with "/" or just take the file name (highlighted above) and put it into a known good imgur URL. You'll end up with this:
https://i.imgur.com/6vY076t.jpg
Or you can run the URL through something like https://www.urldecoder.org/ (I'm not endorsing this site and will not take responsibility if it steals your money).
As for why... only theymos can tell for sure by checking what imgur is returning (some kind of 400 or 500 error page perhaps).