as of today, 2013-Apr-11, when I try to upload to ImgUr I get "An error occured while uploading to Imgur: The remote server returned an error: (403) Forbidden." Yesterday worked fine. It doesn't matter if I use anonymous or authenticated upload. A wait of 2 hours between tests made no difference.
I'm using Greenshot 1.1.1 build 2550 64bit on Windows 7 x64.
I checked the imgur blog (http://imgur.com/blog/) and there's no mention of change in terms of service, but there is a notice from 2 days ago about momentary over capacity issues while doing backups. A regular upload from the browser is fine.