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I was getting this error when trying to upload images. In particular, I was trying to upload and set thumbnails on posts. At first I thought the problem was either: (A) my iPad, or (B) the fact I'm in China.
However, the problem repeated on my PC with VPN on. So, I concluded that it was indeed a WordPress bug. I searched around and found many others were getting the same error. One post suggested a WordPress power cycle.
This worked for me:
- Deactivate all plugins (I deactivated them in bulk)
- Re-instal WordPress (I used the automatic re-instal option)
- Re-activate plugins one-by-one
Now images upload without error and are processed to produce the various sizes, including thumbnail versions.