Imgur is having some trouble I guess.
I can't upload any images on Stack Exchange.
It looks they have some maintenance issues.
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Imgur's upstream provider is having storage issues. They're keeping us updated and will let us know as soon as the issues are fixed. Until then, we've disabled image uploading (meaning that you'll only get the good ol' "enter image url" dialog when clicking the image button in the editor, instead of the upload dialog).
Update: Should be back to working now, at least for
stack.imgur.com. Image uploads are re-enabled.