Asset upload fails with progress bar at full, but images in tempuploads folder
My own fault – I had set a subpath in the field type, so it tried to save to a folder that didn't exist.

I had first thought it was here the asset path was set, and when I figured out I needed asset types, I forgot to delete the subpath.

Now it works! Both with Imagemagick (local) and GD (live server).