I was trying to clone one blog to another using the import/export functionality of WordPress. I had selected to export everything on the original blog, and Download attachments on the clone.
However, I received an error message for each image of the source, saying that the download failed.
The images actually show up, both in the editor and on the site. Obviously both blogs are live.
It is possible to upload arbitrary images to the blog's media library (but I had to increase the file size in nginx manually for some reason).
What is the problem and how can I solve it?