Same issue, and only after upgrading to 4.0. Guido, what plugins did you disable? I’ve stripped my plugins down to what I think are the bare minimums, and would rather not shut anything off.
All: even though it shows an error, the image is still available to drop into your post. So there’s that. You just can’t see the thumbnail. Oddly, I was able dump GIFs in no problem, but .jpgs are causing an issue.