Thread:Project:Support desk/ImageMagick Thumbnail error: "no decode delegate"; "no data returned"

I can't get ImageMagick to generate thumbnails. After a timeout (60 seconds or so), I get a grey box with the following error:

Error creating thumbnail: convert: no decode delegate for this image format `' @ error/constitute.c/ReadImage/532. convert: no decode delegate for this image format `/var/tmp/magick-zeDREKz6' @ error/constitute.c/ReadImage/532. convert: no data returned `http://[wiki URL]/index.php/File:Dan3.jpg' @ error/url.c/ReadURLImage/231.

If I turn off ImageMagick (commenting out "$wgUseImageMagick = true;") it works (via GD), but of course I want to be able to use ImageMagick.

I've confirmed that 'convert' works from the command line, and that $wgImageMagickConvertCommand points to the right place.

Something that would at least get me on the right path: is the root cause of the above error message "no decode delegate" or "no data returned"? Does ReadImage call ReadURLImage, or is it the other way around?

Versions: MediaWiki 1.16.5, PHP 5.3.4, MySQL 5.1.44, OS X 10.6.7, ImageMagick 6.6.9-10